[CP2K:10562] unit

Iain Bethune - UKRI STFC iain.b... at stfc.ac.uk
Mon Jul 23 13:59:42 UTC 2018

Hi Simin,

The forcefield constants in the CHARMM format are expressed in kcal/mol and Angstrom units system (this is the convention that the CHARMM community uses).  CP2K reads a CHARMM-formatted file and converts to CP2K internal units (see https://github.com/cp2k/cp2k/blob/master/cp2k/src/force_fields_ext.F)

!      UNITS for Kb: [(kcal/mol)/(A^2)] to [Eh/(AU^2)]
!      UNITS for Ktheta: [(kcal/mol)/(rad^2)] to [Eh/(rad^2)]
!      UNITS for Kub: [(kcal/mol)/(A^2)] to [Eh/(AU^2)]
!      UNITS for Kphi: [(kcal/mol)] to [Eh]
!      UNITS for Kpsi: [(kcal/mol)/(rad^2)] to [Eh/(rad^2)]

For force-fields that you define explicitly using the MM%FORCEFIELD sub-sections, you can specify whatever dimensionally consistent units you like.  By default CP2K uses atomic units (hartree and bohr).  See e.g. https://manual.cp2k.org/cp2k-5_1-branch/CP2K_INPUT/FORCE_EVAL/MM/FORCEFIELD/BEND.html


- Iain


Iain Bethune
Technical Programme Manager, STFC Hartree Centre

Email: iain.b... at stfc.ac.uk
Twitter: @IainBethune @PrimeGrid @CP2Kproject

Tel: +44 (0)1925 603735
Mob: +44 (0)7598317015
Addr: Hartree Centre, Sci-Tech Daresbury, Keckwick​,​ Warrington, WA4 4AD

> On 23 Jul 2018, at 13:51, simin pahlavi <simin.... at gmail.com> wrote:
> Dear Cp2k users,
> I have a question about units.
> In exercises ( Fist ) parts of cp2k webpage , the units of for example force constants (in separate file for potentials) is kcal.mol/An ^2 without any changing in unit in inp file,  while according to the cp2k manual is supposed to be hartee/boh^2. And same for the other constants and parametrs.
> Does anybody can describe why without, changing the units, cp2k automatically find out that for instance force constants in charm potentials is written in kcal/mol An^2 unit?
> Best regards
> Simin
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