===== Which basis sets and/or pseudopotentials should I use? =====

When in doubt, always use the ''DZVP-MOLOPT-GTH'' basis set and the ''GTH-PBE'' pseudopotential.

In some cases (for example for tin) there is only a short-range basis set available, called ''DZVP-MOLOPT-SR-GTH''.

You can find all available MOLOPT basis sets in ''BASIS_MOLOPT'', which is located in the folder ''$CP2K_DATA_DIR'' (after loading the CP2K module).

===== How do I run CP2K in parallel? =====

The command ''cp2k.sopt'' is the single-process variant of CP2K.

To run CP2K in parallel, you have to use a different executable named ''cp2k.popt'' and prefix that with ''mpirun -n 8'' to run it on 8 CPUs in parallel. So, a complete command line to run CP2K on 8 CPUs would then be:

<code>
$ mpirun -n 8 cp2k.popt -i yourinput.inp -o youroutput.out
</code>


===== How do I obtain a charge density difference? =====

In [[exercises:2016_uzh_cmest:basic_electronic_structure|Exercise 2]] a series of ''*.cube'' files was already produced for MOs.
To obtain the electronic charge density in a ''*.cube'', file the following snippet of input file can be used (see also the [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/DFT/PRINT/E_DENSITY_CUBE.html|reference manual]]):
<code>
&FORCE_EVAL
  &DFT
    &PRINT
      &E_DENSITY_CUBE
      &END E_DENSITY_CUBE
      ...
    &END PRINT
    ... ...
  &END DFT
&END FORCE_EVAL
</code>

The [[tools:cubecruncher|cubecruncher]] tool can be used to obtain charge density differences. A precompiled executable is available on the server ''tcopt3'', its path being ''/users/scaravat/bin/cubecruncher.x''.

The basic usage to obtain a charge density difference is:
<code>
$ /users/scaravat/bin/cubecruncher.x -i input.cube -o output.cube -subtract subsystem.cube
</code>

===== How do I build a slab model for a surface? =====

E.g. by using [[http://jp-minerals.org/vesta/en/|VESTA]].
A tutorial is available [[http://hjklol.mit.edu/content/building-slab-geometries-catalysis-vesta|here]].

Vesta comes preinstalled on ''tcopt3'', provided that you load the proper module.
<code>
$ module load vesta
</code>

===== How do I obtain the stress tensor? =====

You have to add the following snippet to the input file:
<code>
&FORCE_EVAL
  ...
  STRESS_TENSOR ANALYTICAL
  &PRINT
    &STRESS_TENSOR
    &END STRESS_TENSOR
  &END PRINT
  ...
&END FORCE_EVAL
</code>

<note tip>Your calculation should be set up in such a way that forces are being calculated:
e.g. ''RUN_TYPE'' set to ''GEO_OPT'' or ''ENERGY_FORCE''; instead ''RUN_TYPE ENERGY'' won't work!</note>


===== My calculation does not converge =====

==== ... and I use MULTIPLE_UNIT_CELL ====

  * first make sure that you actually need ''MULTIPLE_UNIT_CELL''. In case you want to do band structure calculation, you most definitely do **not** want it
  * if you are sure that you need it, make sure that it is specified two times: once in the [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/SUBSYS/CELL.html#MULTIPLE_UNIT_CELL|''CELL'' section]] and a second time in the [[https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/SUBSYS/TOPOLOGY.html#desc_MULTIPLE_UNIT_CELL|''TOPOLOGY'' section]], otherwise your calculation may or may not converge