howto:xas_tdp

# How to run XAS LR-TDDFT calculations

This a a short tutorial on how to run near-edge X-ray absorption spectroscopy calculations using linear-response TDDFT. The method is implemented in CP2K under the XAS_TDP name. It relies on core-level specific approximations that enables efficient calculations of large and periodic systems. Both K- and L-edge are available. The details of the method can be found in Physical Chemistry Chemical Physics, 2021, DOI: 10.1039/D0CP06164F. Please cite this paper if you were to use the XAS_TDP method for work you publish. Note that XAS LR-TDDFT comes with a correction scheme, described further down this tutorial.

## Brief theory recap

The method is based on 3 main core-specific approximations that boost the calculation efficiency. The first one is the core-valence separation. Due to large differences in energy and localization, core and valence states only weakly couple. Thus, when dealing with XAS, it is customary to simply ignore excitations from valence states.

The second approximation is the sudden approximation, in which the relaxation of electrons beyond the core region is neglected upon excitation of a core electron. Combined with the localized nature of core states, this allows to treat excitations one at a time rather than all at once. This is more efficient in the sense that diagonaling a series of small matrices scales better than diagonalizing a single much larger one.

Finally, a lot of 4-center 2-electron integrals have to be computed. Thanks to the core-valence separation and the sudden approximation, all required integrals involve the core state from which the exciation takes place. This allows for a core-specific resolution of the identity scheme (RI). For the Coulomb integrals:

\begin{equation} (pI|Jq) \approx \sum_{\mu, \nu} \ (pI|\mu) \ (\mu|\nu)^{-1} \ (\nu|Jq) \end{equation}

where $p,q$ represent atomic orbitals (Gaussian type orbitals/GTOs) and $I, J$ core orbtials. For non-zero integrals, $p,I$ and $q,J$ have to overlap. Since $I,J$ are localized core orbitals centered on the same atom, it is sufficient to take a RI basis only made GTOs centered on the excited atom. Note that in case of K-edge spectroscopy, there is only one core state to consider and $I=J$. For L-edge, $I,J$ span all three degenerate $2p$ states. This leads to particularly efficient integral evaluations.

For the exchange-correlation kernel, the RI scheme reads:

\begin{equation} (pI|f_{xc}|Jq) \approx \sum_{\kappa, \lambda, \mu, \nu, } \ (pI|\kappa) \ (\kappa|\lambda)^{-1} \ (\lambda|f_{xc}|\mu) \ (\mu|\nu)^{-1} (\nu|Jq) \end{equation}

where all integrals but $(\lambda|f_{xc}|\mu)$ are the same as for the Coulomb kernel above. Since the RI basis elements $\lambda, \mu$ are centered on the excited atoms, we only need the density in its vicinity. For this, we use a simple projection:

\begin{equation} \label{proj} \begin{aligned} n(\mathbf{r}) &=\sum_\sigma\sum_{pq} P^\sigma_{pq} \ \varphi_p(\mathbf{r}) \varphi_q(\mathbf{r})\\ %\pause &\approx \sum_\sigma \sum_{pq}\sum_{\mu\nu} P^\sigma_{pq} \ (pq\mu) \ S_{\mu\nu}^{-1} \ \chi_\nu(\mathbf{r})\\ &= \sum_\nu d_\nu \ \chi_\nu(\mathbf{r}), \end{aligned} \end{equation}

which turns the density into a linear combination of RI basis elements. This allows for easy and simple numerical integration of $(\lambda|f_{xc}|\mu)$. Note that the quality of the projection may suffer if there are (heavy) atoms close by since their core states may not be well described (GTOs are only sharp at their center). This can be addressed by either using pseudopotentials for the neighbors or adding their RI basis function for the projection.

For these approximations to work, the core states to be excited need to be identified among the Kohn-Sham oritals. They need to have a strong $1s$, $2s$ or $2p$ nature and be well localized. Not fullfilling these conditions will lead to wrong results.

## The XAS_TDP input section

The parameters defining XAS LR-TDDFT calculations are found in the XAS_TDP subsection of DFT. Some external parameters also need to be set to specific values. In particular, RUN_TYPE should be set to ENERGY and DFT%QS%METHOD to GAPW. The combination of GAPW and all-electron basis sets allow for an accurate description of core states.

The most important keywords and subsections of XAS_TDP are:

• DONOR_STATES: which define which core states need to be excited (and where to look for them)
• KERNEL: where the XC functional and exact exchange interaction (for hybrid TDDFT) are defined
• GRID: which defines the integration grids for the xc kernel $(\lambda|f_{xc}|\mu)$

The defaults value of all other keywords are in principle good enough.

Note that the first requirement for XAS LR-TDDFT is that the ground state calculation on which it is based is of good quality.

## Simple examples

Illustrative examples usually tell more than long texts. Below, some typical input examples are displayed with explanations. They should cover most common use cases.

### CO$_2$ molecule (K-edge)

This is a simple C and O K-edge calculation of the CO$_2$ molecule in the gas phase. The annotated input file is displayed below:

CO2.inp
&GLOBAL
PROJECT CO2
RUN_TYPE ENERGY
&END GLOBAL

&FORCE_EVAL
&DFT
BASIS_SET_FILE_NAME EMSL_BASIS_SETS
POTENTIAL_FILE_NAME POTENTIAL
AUTO_BASIS RI_XAS MEDIUM              ! size of automatically generated RI basis

&MGRID
CUTOFF 500
REL_CUTOFF 40
NGRIDS 5
&END MGRID

&QS
METHOD GAPW                         ! It is necesary to use the GAPW method for
&END QS                               ! accurate description of core states

&POISSON
PERIODIC NONE
PSOLVER MT
&END

&SCF
EPS_SCF 1.0E-8
MAX_SCF 30
&END SCF

&XC
&XC_FUNCTIONAL
&LIBXC
FUNCTIONAL HYB_GGA_XC_BHandHLYP
&END LIBXC
&END XC_FUNCTIONAL
&HF
FRACTION 0.5                    ! BHandHLYP functional requires 50% exact exchange
&END HF
&END XC

&XAS_TDP
&DONOR_STATES
DEFINE_EXCITED BY_INDEX         ! We look for states by atom index:
ATOM_LIST 1 2                   ! we want to excite atoms 1 and 2
STATE_TYPES 1s 1s               ! from their 1s core state.
N_SEARCH 3                      ! The 3 lowest energy MOs need to be searched (C1s, O1s, O1s)
LOCALIZE                        ! States need to be actively localized because O atoms are
&END DONOR_STATES                  ! equivalent under symmetry

GRID C 250 500                     ! Integration grid dimensions for C and O excited atoms
GRID O 250 500                     ! there are 250 angular points (Lebedev grid) and 500

&KERNEL
RI_REGION 2.0                   ! Include RI basis elements from atoms within a 2.0 Ang
! sphere radius around the excited atom for the density projection
&XC_FUNCTIONAL
&LIBXC
FUNCTIONAL HYB_GGA_XC_BHandHLYP
&END LIBXC
&END XC_FUNCTIONAL
&EXACT_EXCHANGE
FRACTION 0.5                 ! Definition of the functional for the TDDFT kernel
&END EXACT_EXCHANGE             ! Here (and usually) taken to be the same as the ground state
&END KERNEL
&END XAS_TDP

&END DFT
&SUBSYS
&CELL
ABC 10.0 10.0 10.0
PERIODIC NONE
&END CELL
&COORD
C 5.00 5.00 5.00
O 5.00 5.00 6.16
O 5.00 5.00 3.84
&END COORD
&KIND C
BASIS_SET 6-311G**                ! Using all-electron basis sets and potential is necessary
POTENTIAL ALL                     ! for the correct description of core states
&END KIND
&KIND O
BASIS_SET 6-311G**
POTENTIAL ALL
&END KIND
&END SUBSYS
&END FORCE_EVAL



There are a few points of particular interest in the XAS_TDP section of this input file. When defining the DONOR_STATES subsection, we specify that excited atoms are defined by their index and proceed to list atoms 1 and 2. This is such that excitations take place from the Carbon 1s level and from one of the Oxygen 1s levels. Since both O atoms are equivalent under symmetry, there is no need to run the calculation for both of them (this calculation is small enough and can run on a single processor, this is mostly for illustration purposes).

In the KERNEL subsection, the RI_REGION keyword is set to 2.0. This is such that all atoms within a 2.0 Angstrom radius of the current atom provide RI basis functions for the projection of the density (see last equation of theory recap). This leads to a better quality description, especially for the Carbon atom which has 2 heavier Oxygens around. Note that if the O atoms were described with the use of a pseudopotential, there would be no need for RI_REGION.

In the resulting output file, there is a XAS_TDP section reporting the steps of the calculations. Especially important are the parts related to donor core state identification. For each excited atom/core level combination, a small report about Mulliken population analysis and overlap with pure donor level is printed. For the Carbon 1s:

# Start of calculations for donor state of type 1s for atom   1 of kind C

The following localized MO(s) have been associated with the donor state(s)
based on the overlap with the components of a minimal STO basis:
Spin   MO index     overlap(sum)
1          3          1.00113

The next best overlap for spin 1 is 0.00000 for MO with index    1

Mulliken population analysis retricted to the associated MO(s) yields:
Spin  MO index     charge
1         3      1.002

Both the overlap and the Mulliken charge should be as close to 1.0 as possible. This ensure that the molecular orbital selected is of the correct type (here projection on a C 1s Slater type orbital) and properly localized (there is a full electron associated to this MO, on this atom). If those numbers are lower, something went wrong with the core level identification. This is usually solved by increasing N_SEARCH or/and by usinge the LOCALIZE keyword in case some atoms are equivalent under symmetry.

Spectral information are given in a separate file named CO2.spectrum, where excitation energies are listed with corresponding oscillator strengths, for each excited core level.

### Tetrahedral NaAlO$_2$ (K-edge, periodic)

This example is about crystalline sodium aluminate and illustrates how large periodic structures can be efficiently simulated.

sodal.inp
&GLOBAL
PROJECT  sodal
RUN_TYPE ENERGY
PRINT_LEVEL LOW
&END GLOBAL

&FORCE_EVAL
METHOD QS
&DFT
BASIS_SET_FILE_NAME  BASIS_PCSEG
BASIS_SET_FILE_NAME  BASIS_MOLOPT
POTENTIAL_FILE_NAME  POTENTIAL
AUTO_BASIS RI_XAS MEDIUM

&QS
METHOD GAPW                         ! GAPW is necessary for core states
&END QS

&AUXILIARY_DENSITY_MATRIX_METHOD       ! The ADMM methog greatly accelerated the ground state calculation
ADMM_PURIFICATION_METHOD NONE       ! This is the simplest ADMM scheme and has proven to work well
&END AUXILIARY_DENSITY_MATRIX_METHOD

&SCF
MAX_SCF    30
EPS_SCF    1.0E-06

&OT
MINIMIZER DIIS
PRECONDITIONER FULL_ALL
&END OT
&OUTER_SCF
MAX_SCF    6
EPS_SCF    1.0E-06
&END
&END SCF

&MGRID
CUTOFF 400
REL_CUTOFF 40
NGRIDS 5
&END

&XC
&XC_FUNCTIONAL PBE               ! This is the PBEh functional with 45% HFX
&PBE                          ! Large fraction of HFX are ususally needed for XAS LR-TDDFT
SCALE_X 0.55
&END
&END XC_FUNCTIONAL

&HF
FRACTION 0.45
&INTERACTION_POTENTIAL
POTENTIAL_TYPE TRUNCATED   ! The tuncated Coulomb potential has to be used in PBCs
CUTOFF_RADIUS 5.0          ! with a cutoff radius lower than half the cell size
&END INTERACTION_POTENTIAL
&SCREENING
EPS_SCHWARZ 1.0E-6         ! Screening HFX integrals boosts performance
&END SCREENING
&END HF
&END XC

&XAS_TDP
&DONOR_STATES
DEFINE_EXCITED BY_KIND        ! We define the excited atoms by kind, which is named Alx here
KIND_LIST Alx                 ! There is only one Alx atom in the coordinates since all Al
STATE_TYPES 1s                ! atoms are equivalent under symmetry. The Alx atom is the only
N_SEARCH 1                    ! one decribed at all-electron level, which is why we use
&END DONOR_STATES                ! N_SEARCH = 1. There is also no need to LOCALIZE

TAMM_DANCOFF                     ! TDA is turned on by default, but we make it explicit here
GRID Alx 150 300
ENERGY_RANGE 20.0                ! This means that we onluy solve for excitation energies that are
! up to 20.0 eV above the first energy
&OT_SOLVER
MINIMIZER DIIS                ! The iterative OT solver is typically more efficient than
EPS_ITER 1.0E-4               ! full diagonalization for large systems
&END OT_SOLVER

&KERNEL
&XC_FUNCTIONAL PBE
&PBE
SCALE_X 0.55
&END
&END XC_FUNCTIONAL

&EXACT_EXCHANGE
OPERATOR TRUNCATED
RANGE  5.0
SCALE 0.45
&END EXACT_EXCHANGE
&END KERNEL
&END XAS_TDP
&END DFT

&SUBSYS
&CELL
ABC 10.467947   10.651128   14.393541
&END CELL
&TOPOLOGY
COORD_FILE_NAME sodal.xyz
COORD_FILE_FORMAT xyz
&END TOPOLOGY
&KIND O
BASIS_SET DZVP-MOLOPT-SR-GTH
BASIS_SET AUX_FIT FIT3
POTENTIAL GTH-PBE
&END KIND
&KIND Na
ELEMENT Na
BASIS_SET DZVP-MOLOPT-SR-GTH
BASIS_SET AUX_FIT FIT3
POTENTIAL GTH-PBE
&END
&KIND Al
BASIS_SET DZVP-MOLOPT-SR-GTH
BASIS_SET AUX_FIT FIT3
POTENTIAL GTH-PBE
&END
&KIND Alx                          ! All atoms but the single Alx are described using pseudopotentials
ELEMENT Al                       ! This greatly reduces the number of basis function and the cost of
BASIS_SET pcseg-2                ! the calculation in general. AUX_FIT basis sets are for ADMM
POTENTIAL ALL
&END
&END SUBSYS
&END FORCE_EVAL



There are many performance oriented keywords and subsection in the above input. Most importantly, only one atom is treated at the all-electron level (the one atom from which the excitation takes place), all other are described using pseudopotentials. Also quite important is the usage of the ADMM method. This allows for very efficient evaluation of the HFX energy in the ground state calculation. Finally, the OT iterative solver is used. Since only a handful of eigenvalues are calculated (those within 20.0 eV of the first excitation energy), this scales much better than a full digonalization. Note that the RI_REGION keyword is absent (it is set to 0 by default). Since the neighbors of the excited Al atom are described with pseudopotentials, there is no need for extra RI basis function for the projection of the density.

This input file would generate a spectrum such as the one visible on figure 4 of the reference work. This is a much larger calculation than the first example though and would require a few hours on 20-30 processors (mostly to converge the SCF). In you are interested in reproducing this result, input, geometry and pcseg-2/admm-2 basis sets are available here.

### TiCl$_4$ molecule (L-edge + SOC)

This example covers L-edge spectroscopy with the addition of spin-orbit coupling.

TiCl4.inp
&GLOBAL
PROJECT TiCl4
PRINT_LEVEL LOW
RUN_TYPE ENERGY
&END GLOBAL
&FORCE_EVAL
&DFT
BASIS_SET_FILE_NAME  BASIS_DEF2-TZVPD
POTENTIAL_FILE_NAME  POTENTIAL
AUTO_BASIS RI_XAS    LARGE

&POISSON
PERIODIC NONE
PSOLVER MT
&END POISSON
&QS
METHOD GAPW
&END QS

&MGRID
CUTOFF 800
REL_CUTOFF 50
NGRIDS 5
&END

&SCF
EPS_SCF 1.0E-8
MAX_SCF 200
&MIXING
METHOD BROYDEN_MIXING
ALPHA 0.2
BETA 1.5
NBROYDEN 8
&END MIXING
&END SCF

&XC
&XC_FUNCTIONAL
&LIBXC
FUNCTIONAL HYB_GGA_XC_B3LYP
&END LIBXC
&END XC_FUNCTIONAL
&HF
FRACTION 0.2
&END HF
&END XC

&XAS_TDP
&DONOR_STATES
DEFINE_EXCITED BY_KIND
KIND_LIST Ti
STATE_TYPES 2p             ! 2p core state for L-edge
&END DONOR_STATES             ! No need to LOCALIZE since only one Ti atom

TAMM_DANCOFF FALSE            ! TDA is on by default, get full TDDFT like this
DIPOLE_FORM LENGTH

GRID Ti 500 1000              ! This is a fairly dense grid

EXCITATIONS RCS_SINGLET       ! For SOC calculations in closed-shell system, these 3 keywords
EXCITATIONS RCS_TRIPLET       ! are required. Singlet and triplet excitation are coupled together
SOC                           ! with the SOC hamiltonian

&KERNEL
&XC_FUNCTIONAL
&LIBXC
FUNCTIONAL HYB_GGA_XC_B3LYP
&END LIBXC
&END XC_FUNCTIONAL
&EXACT_EXCHANGE
FRACTION 0.2
&END EXACT_EXCHANGE
&END KERNEL
&END XAS_TDP

&END DFT
&SUBSYS
&KIND Cl
BASIS_SET def2-TZVPD
POTENTIAL ALL
RADIAL_GRID 80                ! The GAPW grids are also used to evaluate the SOC operator
LEBEDEV_GRID 120              ! it is good practice to use sligthly larger ones than the default
&END KIND
&KIND Ti
BASIS_SET def2-TZVPD
POTENTIAL ALL
LEBEDEV_GRID 120
&END KIND
&CELL
ABC 10.0 10.0 10.0
PERIODIC NONE
&END CELL
&TOPOLOGY
COORD_FILE_FORMAT XYZ
COORD_FILE_NAME TiCl4.xyz
&CENTER_COORDINATES
&END CENTER_COORDINATES
&END TOPOLOGY
&END SUBSYS
&END FORCE_EVAL


The structure of the input file is not very different from the CO$_2$ example. Notable differences are the DONOR_STATES subsection where 2p states are specified and the combinations of the EXCITATIONS and SOC keywords. Indeed, the way spin-orbit coupling is treated in XAS_TDP is by coupling together singlet and triplet excitation via the ZORA SOC Hamiltionian.

Note that this calculation is meant to be a benchmark calculation, hence the overall larger GRIDs and CUTOFFs. The result can be seen in the reference work, figure 1 a). The calculation takes about 10 minutes on 4 cores. All necessary files are available here.

In the output file, the donor state identification yields overlaps that are greater than one. This is due to the degenerate nature of 2p states. The candidate Kohn-Sham orbital is projected on 3 STOs for 2px, 2py and 2pz. To avoid cancelling contributions, the sum of the absolute overlap is taken.

  # Start of calculations for donor state of type 2p for atom   1 of kind Ti

The following canonical MO(s) have been associated with the donor state(s)
based on the overlap with the components of a minimal STO basis:
Spin   MO index     overlap(sum)
1          7          1.36751
1          8          1.36751
1          9          0.99786

The next best overlap for spin 1 is 0.06653 for MO with index   27

Mulliken population analysis retricted to the associated MO(s) yields:
Spin  MO index     charge
1         7      1.000
1         8      1.000
1         9      1.000

## FAQ

### Which functional and basis sets to use ?

Hybrid functionals with high fraction of Hartree-Fock exchange are know to perform well for core spectroscopy. PBEh($\alpha=0.45$) and BHandHLYP have had success with this particular implementation. In periodic boundary conditions, the truncated Coulomb operator should be used (with truncation radius < half cell parameter).

For appropriate description of core states, all-electron basis sets should be used for the excited atom(s). MOLOPT basis sets and pseudopotentials can be used on all other atoms. There exist core specific basis such as pcX-n and cc-pCVXZ, but their usage is not necessary (based on basis set convergence studies on small molecules). Note that the pcseg-n basis sets are nice to use as they come with their own ADMM basis.

### How do I make my calculation more accurate ?

The first necessity is to have a good ground state calculation. Thus, any change of paramter improving the SCF will reflect on the quality of the LR-TDDFT calculation.

Within XAS_TDP, a few parameters may play a role:

• EPS_FILTER and EPS_PGF are used for screening. Lowering those will result in slower but more accurate calculations
• Increasing the GRID dimensions will improve the quality of the numerical integration of the XC kernel. The upper limit for the number of angular points is 974. There is not upper limit for the radial point, but performance may suffer if too large. Usually, something like GRID C 250 500 is sufficient.
• Increasing the RI_REGION in the KERNEL subsection will lead to a more accurate projection of the density on the RI basis. Basis functions centered on atoms within the region (defined by a sphere around the excited atom) are added for the projection. Increasing this parameter should come together with denser GRID.

By default, the RI basis used for the integral and the projection is autamatically generated. The quality of the RI basis can be changed via the AUTO_BASIS keyword in the DFT section. To improve from the default MEDIUM size, one can used: AUTO_BASIS RI_XAS LARGE/HUGE. Note that an external RI basis set can also be provided.

### How do I make my calculation faster ?

All points mentioned above in the accuracy section can also be tweaked for performence. In general, lowering accuracy will lead to faster calculations.

For large systems, it is recommanded to use the OT iterative solver rather than the default full digonalization. This should improve the scaling of the method. See the NaAlO$_2$ example.

The Tamm-Dancoff approximation is well established and generally yields results as good as full TDDFT. It is moreover much cheaper than the latter. It is turned on by default, but you may want to make sure it is enabled.

The use of ADMM is highly recommanded for large systems, where the ground state HFX evaluation is the main bottleneck. It is also recommanded to use pseudopotentials on all atoms that are not excited as all-electron basis set tend to be large. If there exist no proper ADMM basis for the all-electron basis used for the excited atom, you may use the full basis as AUX_FIT. If the latter is very diffuse, it may be beneficial to remove the most diffuse elements.

The code is also both MPI and OMP parallelized. Using more core will, to a certain degree, speedup your calculations as well.

### How do I plot a spectrum from the *.spectrum output file

For each donor state in the system, the *.spectrum file contains a list of excitation energies and corresponding oscillator strengths. This yields a stick spectrum which needs to be artificially broadened to match experiments. This is typically done using Gaussian of Lorentzian functions. Note that in case of spin-orbit calculation at the L-edge, results for the singlet, triplet and SOC excitations are given.

Remember that XAS LR-TDDFT produces an accurate spectrum, but it is usually wrongly positioned on the energy axis. Again, to match experiment, a rigid shift must be applied to the result.

### My calculation yields a wrong/unphysical result, what do I do ?

Assuming that the ground state calculation is sound and well converged, there are two main causes for failure.

The proper core state was not found/identified. In the XAS_TDP part of the output file, look for the Mulliken population analysis and the overlap with a STO basis. Both quantities should close to 1.0. If they are not, there is a problem with the selected core state. You may want to increase the value of N_SEARCH in DONOR_STATES to scan additional Kohn-Sham orbitals. If there are multiple atoms that are equivalent under symmetry, make sure to use the LOCALIZE keyword of DONOR_STATES as well.

The numerical integration of the XC kernel $(\lambda|f_{xc}|\mu)$ is not accurate enough. In this case, you may want to increase the density of the integration grid with the GRID keyword, increase the RI_REGION and/or increase the quality of the genenerated RI basis set. Note that using an external RI basis set may also help as the basis generation scheme may fail (For example: def2-QZVP for Zn, use def2-QZVP-RIFIT in this case).

Finally, keep in mind that calculated spectra need to be rigidly shifted by some energy to match experiment.

# First-principles correction scheme

As mentioned above, XAS LR-TDDFT results need to be rigidly shifted to match experiments. This is due to self-interaction error and the lack of orbital relaxation upon the creation of the core hole. An ab-initio correction scheme was developed to address these issues. Theory and benchmarks were published in The Journal of Chemical Physics 2021, DOI: 10.1063/5.0058124. Please cite this paper if you were to use this method.

### Brief theory recap

XAS LR-TDDFT yield excitation energies as correction to ground state Kohn-Sham orbital energy differences, namely:

$$\omega = \varepsilon_a - \varepsilon_I + \Delta_{xc},$$

where $\varepsilon_a$ is the orbital energy of a virtual MO and $\varepsilon_I$ the energy of the donor core MO. Under Koopman's condition, these energies are interpreted as the electron affinity and and the ionization potential (IP). However, DFT is notoriously bad at predicting accurate absolute orbital eigenvalues. Therefore, and because $|\varepsilon_I| >> |\varepsilon_a|$, excitation energies are expected to be widely improved if the DFT energy $\varepsilon_I$ were to be replace by an accurate value of the IP.

The IP can be accurately calculated using the second-order electron propagator equation:

$$\text{IP}_I = -\varepsilon_I - \frac{1}{2} \sum_{ajk}\frac{|\langle Ia||jk\rangle|^2}{-\text{IP}_I + \varepsilon_a -\varepsilon_j -\varepsilon_k} - \frac{1}{2}\sum_{abj}\frac{|\langle Ij||ab\rangle|^2}{-\text{IP}_I + \varepsilon_j - \varepsilon_a - \varepsilon_b}$$

where $a, b$ refer to virtual Hartree-Fock spin-orbitals and $j,k$ to occupied HF spin-orbitals. The DFT generalization of this theory is known as GW2X. It involves calculating the Generalized Fock matrix and the rotation of the occupied and virtual DFT orbitals separately, such that they become pseudocanonical. Alternatively, the diagonal elements of the generalized Fock matrix can be used as approximations for the orbital energies (thus saving on the orbital rotation). This is known as the GW2X* method.

### The GW2X input subsection

The parameters defining the GW2X correction to XAS LR-TDDFT are found in the GW2X subsection of XAS_TDP. GW2X will only work with hybrid functionals (or full Hartree-Fock), as the machinery necessary for the calculation of the generalized Fock matrix is not available otherwise.

There are not many parameters to set for the GW2X correction. Simply adding an empty &GW2X subsection is usually enough. The electron propagator equation is solved iteratively with a Newton-Raphson scheme. EPS_GW2X controls the convergence threshold and MAX_GW2X_ITER the maximum number of iterations allowed. The PSEUDO_CANONICAL keyword controls whether the original GW2X scheme or its simplified GW2X* version is run (by default, the original GW2X is on). C_SS and C_OS allow to scale the same- and opposite-spin components (as in SOS- and SCS-MP2). Finally, if XPS_ONLY is set, only the core IP is calculated and the XAS LR-TDDFT calculation is skipped.

### Simple examples

#### OCS molecule (L-edge + SOC)

This example covers GW2X corrected L-edge spectroscopy with spin-orbit coupling.

OCS.inp
&GLOBAL
PROJECT OCS
PRINT_LEVEL MEDIUM
RUN_TYPE ENERGY
&END GLOBAL
&FORCE_EVAL
METHOD Quickstep
&DFT
BASIS_SET_FILE_NAME BASIS_GW2X
POTENTIAL_FILE_NAME POTENTIAL
AUTO_BASIS RI_XAS MEDIUM

&MGRID
CUTOFF 800
REL_CUTOFF 50
NGRIDS 5
&END MGRID
&QS
METHOD GAPW
&END QS

&POISSON
PERIODIC NONE
PSOLVER MT
&END

&SCF
EPS_SCF 1.0E-8
MAX_SCF 50
&END SCF

&XC
&XC_FUNCTIONAL                ! The PBEh(45%) functional
&LIBXC
FUNCTIONAL GGA_C_PBE
&END LIBXC
&LIBXC
FUNCTIONAL GGA_X_PBE
SCALE 0.55
&END LIBXC
&END XC_FUNCTIONAL

&HF
FRACTION 0.45
&END HF
&END XC

&XAS_TDP
&DONOR_STATES
DEFINE_EXCITED BY_KIND
KIND_LIST S
STATE_TYPES 2p          ! Need to look for the S 2p states within the 7 MOs with lowest energy;
N_SEARCH 7              ! one S 1s, one S 2s, three S 2s , one C 1s and one O 1s
LOCALIZE                ! Localization is required
&END DONOR_STATES

EXCITATIONS RCS_SINGLET
EXCITATIONS RCS_TRIPLET
SOC

GRID S  300 500

N_EXCITED 150
TAMM_DANCOFF

&GW2X                      ! This is the only difference in the input file with respect to a
&END GW2X                  ! standard XAS_TDP calculation (defaults parameters are used)

&KERNEL
RI_REGION 3.0
&XC_FUNCTIONAL
&LIBXC
FUNCTIONAL GGA_C_PBE
&END LIBXC
&LIBXC
FUNCTIONAL GGA_X_PBE
SCALE 0.55
&END LIBXC
&END XC_FUNCTIONAL
&EXACT_EXCHANGE
FRACTION 0.45
&END EXACT_EXCHANGE
&END KERNEL

&END XAS_TDP
&END DFT
&SUBSYS
&CELL
ABC 10.0 10.0 10.0
PERIODIC NONE
&END CELL
&COORD
C         5.0000000209        4.9999999724        5.2021372095
O         5.0000000094        5.0000000290        6.3579624316
S         5.0000000207        5.0000000007        3.6399034216
&END COORD
&KIND C
BASIS_SET aug-pcX-2
POTENTIAL ALL
&END KIND
&KIND O
BASIS_SET aug-pcX-2
POTENTIAL ALL
&END KIND
&KIND S
BASIS_SET aug-pcX-2
POTENTIAL ALL
&END KIND
&END SUBSYS
&END FORCE_EVAL


The only difference between the above input file and that of a standard XAS LR-TDDFT calculation is the addition of the &GW2X subsection. In this case, only default parameters are used, which corresponds to the original GW2X scheme with a convergence threshold of 0.01 eV. Note that the core specific all-electron aug-pcX-2 basis set is used (triple zeta quality). This inputs corresponds to an entry of table II in the reference paper, although slacker parameters are used here (in order to make this tutorial cheap and easy to run, this particular calculations takes ~2 minutes on 4 cores).

In the output file, the correction for each S $2p$ is displayed. Note that the correction amounts to a shift of 1.9 eV compared to standard XAS LR-TDDFT, leading to a first singlet excitation energy of 164.4 eV (at the L$_3$ edge). This fits experimental results within 0.1 eV. thus clearly improving the XAS LR-TDDFT result. Note that the core IPs, including spin-orbit coupling effects, are also provided. These can be directly used to produce a XPS spectrum. The content of the OCS.spectrum file yields the corrected spectrum directly.

    - GW2X correction for donor MO with spin  1 and MO index    5:
iteration                convergence (eV)
1                       10.047536
2                        1.237503
3                        0.014416
4                       -0.000000

Final GW2X shift for this donor MO (eV):   1.927146

- GW2X correction for donor MO with spin  1 and MO index    6:
iteration                convergence (eV)
1                        6.197650
2                        4.963008
3                        0.241838
4                        0.000439

Final GW2X shift for this donor MO (eV):   1.907648

- GW2X correction for donor MO with spin  1 and MO index    7:
iteration                convergence (eV)
1                        6.197650
2                        4.963008
3                        0.241838
4                        0.000439

Final GW2X shift for this donor MO (eV):   1.907648

Calculations done:

First singlet XAS excitation energy (eV):                165.014087
First triplet XAS excitation energy (eV):                164.681850
First SOC XAS excitation energy (eV):                    164.396537

Ionization potentials for XPS (GW2X + SOC):              170.602279
169.457339
169.367465


#### Solid NH$_3$ (K-edge, periodic)

This is a much larger example of a periodic system, namely solid ammonia. This example is much heavier to run (~45 minutes on 24 cores).

&GLOBAL
PROJECT NH3
RUN_TYPE ENERGY
PRINT_LEVEL MEDIUM
&END GLOBAL
&FORCE_EVAL
METHOD QS
&DFT
BASIS_SET_FILE_NAME BASIS_GW2X
BASIS_SET_FILE_NAME BASIS_MOLOPT
POTENTIAL_FILE_NAME POTENTIAL
AUTO_BASIS RI_XAS MEDIUM

&QS
METHOD GAPW
&END QS

&MGRID
CUTOFF 600
REL_CUTOFF 50
NGRIDS 5
&END MGRID

&SCF
SCF_GUESS RESTART
EPS_SCF 1.0E-8
MAX_SCF 30

&OT
MINIMIZER CG
PRECONDITIONER FULL_ALL
&END OT

&OUTER_SCF
MAX_SCF 6
EPS_SCF 1.0E-8
&END OUTER_SCF

&END SCF

&AUXILIARY_DENSITY_MATRIX_METHOD
&END AUXILIARY_DENSITY_MATRIX_METHOD

&XC
&XC_FUNCTIONAL
&LIBXC
FUNCTIONAL GGA_X_PBE
SCALE 0.55
&END
&LIBXC
FUNCTIONAL GGA_C_PBE
&END
&END XC_FUNCTIONAL
&HF
FRACTION 0.45
&INTERACTION_POTENTIAL
POTENTIAL_TYPE TRUNCATED
&END INTERACTION_POTENTIAL
&END HF
&END XC

&XAS_TDP
&DONOR_STATES
DEFINE_EXCITED BY_KIND
KIND_LIST Nx
STATE_TYPES 1s
N_SEARCH 1
LOCALIZE
&END DONOR_STATES

TAMM_DANCOFF
GRID Nx 300 500
E_RANGE 30.0

&GW2X
&END

&KERNEL
&XC_FUNCTIONAL
&LIBXC
FUNCTIONAL GGA_X_PBE
SCALE 0.55
&END
&LIBXC
FUNCTIONAL GGA_C_PBE
&END
&END XC_FUNCTIONAL
&EXACT_EXCHANGE
OPERATOR TRUNCATED
FRACTION 0.45
&END EXACT_EXCHANGE
&END KERNEL

&END XAS_TDP
&END DFT
&SUBSYS
&CELL
ABC   10.016118  10.016118  10.016118
&END CELL
&TOPOLOGY
COORD_FILE_FORMAT XYZ
COORD_FILE_NAME NH3.xyz
&END TOPOLOGY
&KIND H
BASIS_SET DZVP-MOLOPT-SR-GTH
BASIS_SET AUX_FIT FIT3
POTENTIAL GTH-PBE
&END KIND
&KIND N
BASIS_SET DZVP-MOLOPT-SR-GTH
BASIS_SET AUX_FIT FIT3
POTENTIAL GTH-PBE
&END KIND
&KIND Nx
ELEMENT N
BASIS_SET aug-pcseg-2
POTENTIAL ALL
&END KIND
&END SUBSYS
&END FORCE_EVAL

Again, the only difference with respect to a standard XAS-LRTDDFT input file is the &GW2X subsection. This input file corresponds exactly to figure 3 a) of the reference papaer. In this case, the GW2X correction amounts to a blue shift of 3.7 eV, aligning the calculated spectrum to the experimental one remarkably well. The NH3 structure file as well as the necessary basis set file (also for the OCS example) are available here.

### FAQ

#### How can I make the GW2X correction run faster ?

The GW2X correction scheme scales cubically with the number of MOs in the system. Therefore, the best way to improve performance is to reduce that number. Because an accurate description of the core region is only necessary for the exited atoms, all other atoms can be described with pseudopotentials. This drastically reduces the number of MOs since only valence states are kept. In the solid NH3 example above, all nitrogen atoms are equivalent under symmetry. Therefore, their individual contribution to the XAS spectrum is bound to be the same. This allows for the description of a single nitrogen atom at the all-electron level, while all others (and the hydrogens) use pseudopotentials. Note that the ADMM approximation is also utilized. This greatly reduces the cost of the underlying hybrid DFT calculation, as well as the evaluation of the generalized Fock matrix as required by GW2X.

#### Why don't I get the absolute core IP in periodic systems ?

For molecules in non-periodic boundary conditions, the potential is such that it is zero far away. In the periodic case, the zero is ill defined. As a consequence, all Kohn-Sham eigenvalues end up shifted by some unknown, constant amount. Therefore, their absolute values and that of the calculated IP cannot be interpreted in a physical manner. However, the correction scheme depends on the difference $|\varepsilon_a-\varepsilon_I|$, where the shift cancels out.

#### Why is the LOCALIZE keyword required ?

In order to efficiently evaluate the antisymmetric integrals of the type $\langle Ia || jk \rangle$, the same local RI scheme as XAS_TDP is used. Therefore, the core state $I$ needs to be local in space. However, the rotation required to get the pseudocanonical orbitals needed for the original GW2X scheme may break this localization, provided that there are other equivalent atoms in the system. To prevent that from happening, all core states localized on other atoms are ignored for the rotation and the subsequent IP calculation. This has negligible impact since core states belonging to different atoms only weakly interact. It is however important to keep the value of the LOCALIZE keyword to a minimum to insure that only core states are ignored. 