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--- exercises:2016_uzh_cmest:electronic_structure_dft [2016/10/06 08:23] – created tmueller
+++ exercises:2016_uzh_cmest:electronic_structure_dft [2016/10/06 14:06] – [1. Step: Changing the method in the configuration file] tmueller
@@ Line 2: / Line 2: @@
 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: Running a DFT calculation =====
+Create a new directory for this exercise and create an input input file using the following content:
+<code - 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
+</code>
+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 ''KIND''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 ''$CP2K_DATA_DIR''. Change to this directory and take a look at the file:
+<code>
+$ cd $CP2K_DATA_DIR
+$ less BASIS_SET
+$ less GTH_POTENTIALS
+</code>
+In the basis sets you will find entries like:
+<code>
+[...]
+H DZVP-GTH-PADE
+  0  0  4  2
+.3744350009  -0.0238943732   0.0000000000
+.8058681460  -0.1397943259   0.0000000000
+.4852531032  -0.2530970874   0.0000000000
+.1658235797  -0.6955307423   1.0000000000
+  1  1  1  1
+.7000000000   1.0000000000
+[...]
+</code>
+while the pseudopotentials file contains something like:
+<code>
+H GTH-PADE-q1 GTH-LDA-q1 GTH-PADE GTH-LDA
+.20000000    2    -4.18023680     0.72507482
+</code>
+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?