[CP2K-user] [CP2K:16895] Re: What is the precision on forces emitted by CP2K's energy forces job?

[Matt D]

Hi Alex,

The quality of the forces you calculate will depend on the accuracy to 
which the SCF problem is solved. The tolerance for the latter is given by 
EPS_SCF - a typical value for EPS_SCF during a geometry optimisation would 
be around 1e-7 Ha. Another important parameter for the SCF tolerance is 
EPS_DEFAULT, this should be decreased from the default value of 1e-10 to 
1e-14 to complement the value of 1e-7 for EPS_SCF. The error in the forces 
with respect to these parameters should be sqrt(EPS_SCF) < error on force; 
and sqrt(EPS_DEFAULT)  < EPS_SCF.

The default units in CP2K are always atomic units, so the force is in 
Hartree/Bohr. If you want to change the units you put in the input, this is 
possible but I have forgotten the exact syntax. I think it is something 
like:

KEYWORD [unit] 0.025

and the "unit" is replaced by one of the units in this list:

https://manual.cp2k.org/trunk/units.html

Best wishes,
Matt

On Monday, 25 April 2022 at 21:56:44 UTC+1 alexvd... at gmail.com wrote:

> Hi,
>
> I'm curious what the precision on forces in a cp2k energy_forces job. In 
> particular, how does it depend on various input variables, such as EPS_SCF? 
>
> Also, in the forces section, the printout says the units are in atomic 
> units. Is that also the default unit for all force dependent convergence 
> criteria, such as in a NEB job?
>
> Thanks,
> Alex
>

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