In this exercise, you will perform again an electronic structure calculation (of Ethene), but this time using Density Functional Theory and different functionals.

Create a new directory for this exercise and create an input input file using the following content:

ethene_LDA.inp

&GLOBAL
  PROJECT ethene
  RUN_TYPE ENERGY
  PRINT_LEVEL MEDIUM
&END GLOBAL

&FORCE_EVAL
  METHOD Quickstep              ! Electronic structure method (DFT,...)
  &DFT
    BASIS_SET_FILE_NAME  BASIS_SET
    POTENTIAL_FILE_NAME  GTH_POTENTIALS

    &POISSON                    ! Solver requested for non periodic calculations
      PERIODIC NONE
      PSOLVER  WAVELET          ! Type of solver
    &END POISSON
    &SCF                        ! Parameters controlling the convergence of the scf. This section should not be changed. 
      SCF_GUESS ATOMIC
      EPS_SCF 1.0E-6
      MAX_SCF 300
    &END SCF
    &XC                        ! Parametes needed to compute the electronic exchange potential 
      &XC_FUNCTIONAL PADE
      &END XC_FUNCTIONAL
    &END XC
  &END DFT

  &SUBSYS
    &CELL
      ABC 10 10 10
      PERIODIC NONE              ! Non periodic calculations. That's why the POISSON section is needed 
    &END CELL
    &TOPOLOGY                    ! Section used to center the atomic coordinates in the given box. Useful for big molecules
      &CENTER_COORDINATES
      &END
    &END
    &COORD
    C         -2.15324        3.98235        0.00126
    C         -0.83403        4.16252       -0.00140
    H         -0.25355        3.95641        0.89185
    H         -0.33362        4.51626       -0.89682
    H         -2.65364        3.62861        0.89669
    H         -2.73371        4.18846       -0.89198
    &END COORD
    &KIND H
      ELEMENT H
      BASIS_SET DZVP-GTH-PADE
      POTENTIAL GTH-PADE-q1
    &END KIND
    &KIND C
      ELEMENT C
      BASIS_SET DZVP-GTH-PADE
      POTENTIAL GTH-PADE-q4
    &END KIND
  &END SUBSYS
&END FORCE_EVAL

Comparing this input file to the one from the previous exercise, we notice a couple of things:

• the HF section is missing: this is obvious since we are doing a pure DFT calculation now
• the parameter for the XC_FUNCTIONAL section is set to PADE (which is a synonym für LDA) instead of NONE, meaning that we are going to do a DFT calculation using the Local Density Approximation
• the specifications for the BASIS and the POTENTIAL for the different atom KINDs has changed

As you have seen in the lecture, one has to select a basis set for doing calculations efficiently. Furthermore we approximate the core electrons of an atom by a common pseudopotential instead of calculating them explicitly, reducing the computational complexity even further.

CP2K comes with a number of files, specifying the respective coefficients. Which one of those files is going to be used to lookup the basis sets/pseudopotentials is defined using the BASIS_SET_FILE_NAME and POTENTIAL_FILE_NAME inside the &DFT section. In the &KIND section then one has only to specify an identifier for an entry in the respective file.

The files are located in $CP2K_DATA_DIR. Change to this directory and take a look at the file:

$ cd $CP2K_DATA_DIR
$ less BASIS_SET
$ less GTH_POTENTIALS

In the basis sets you will find entries like:

[...]
H DZVP-GTH-PADE
  2
  1  0  0  4  2
        8.3744350009  -0.0238943732   0.0000000000
        1.8058681460  -0.1397943259   0.0000000000
        0.4852531032  -0.2530970874   0.0000000000
        0.1658235797  -0.6955307423   1.0000000000
  2  1  1  1  1
        0.7000000000   1.0000000000
[...]

while the pseudopotentials file contains something like:

H GTH-PADE-q1 GTH-LDA-q1 GTH-PADE GTH-LDA
    1
     0.20000000    2    -4.18023680     0.72507482
    0

Now return to the previous (exercise) directory ($ cd -) and run the simulation. Compare the energy calculated using DFT-LDA ($ grep 'ENERGY|' ethene_LDA.out) to the one calculated in the last exercise using Hartree-Fock. What do you observe? What does that mean?