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--- howto:xas_tdp [2021/08/02 15:36] – [Brief theory recap] abussy
+++ howto:xas_tdp [2021/08/16 14:48] – [Tetrahedral NaAlO$_2$ (K-edge, periodic)] abussy
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-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.
+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 [[ https://pubs.rsc.org/en/Content/ArticleLanding/2021/CP/D0CP06164F#!divAbstract | 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 {{ :howto:sodal.zip | here}}.
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 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 [[https://doi.org/10.1063/5.0058124|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 [[https://doi.org/10.1016/s0301-0104(97)00111-0|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 yields the corrected spectrum directly.
+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 [[https://doi.org/10.1016/s0301-0104(97)00111-0|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.
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 === 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.
+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.