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Electronic structure calculation using DFT
In this exercise, you will perform again an electronic structure calculation (of Ethene), but this time using Density Functional Theory and different functionals.
1. Step: Changing the method in the configuration file
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 toPADE
(which is a synonym fürLDA
) instead ofNONE
, meaning that we are going to do a DFT calculation using the Local Density Approximation - the specifications for the
BASIS
and thePOTENTIAL
for the different atomKIND
s 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