# CP2K Open Source Molecular Dynamics

### Sidebar

#### For Developers

exercises:2017_uzh_cmest:faq

## 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.

## How do I obtain a charge density difference?

In 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 reference manual):

&FORCE_EVAL
&DFT
&PRINT
&E_DENSITY_CUBE
&END E_DENSITY_CUBE
...
&END PRINT
... ...
&END DFT
&END FORCE_EVAL

The 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:

## How do I obtain the stress tensor?

You have to add the following snippet to the input file:

&FORCE_EVAL
...
STRESS_TENSOR ANALYTICAL
&PRINT
&STRESS_TENSOR
&END STRESS_TENSOR
&END PRINT
...
&END FORCE_EVAL
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!

## 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 ''CELL'' section and a second time in the ''TOPOLOGY'' section, otherwise your calculation may or may not converge

## Error after changing coordinates

If you get an error after copy and pasting coordinates from a website/service/other file into a CP2K input file and you get an error afterwards when trying to run it, please make sure that you are only using spaces as delimiters between the atomic kind and each of the coordinates.

exercises/2017_uzh_cmest/faq.txt · Last modified: 2020/08/21 10:15 (external edit)