The total force calculated by CP2K is not zero

[liu dongfei]

Hi all, I’m currently simulating a pure water system with CP2K. Despite configuring appropriate convergence criteria (e.g., CUTOFF, REL_CUTOFF, and EPS_SCF), the total forces from the calculation do not converge to zero. Attached is my input configuration. 

Could anyone provide insights into potential causes or recommend literature discussing this specific issue? 

Thanks in advance!

[Jürg Hutter]

Hi

inaccurate numerics can be a source for forces not summing up to zero. However, I assume
(we don't know actually, as you don't share the forces) that the residual force is due to the
'ripple effect'. CP2K uses aequidistant grids to calculate XC energies. For such grids there is no
longer any translational invariance.

regards
JH

________________________________________
From: cp...@googlegroups.com <cp...@googlegroups.com> on behalf of liu dongfei <liud...@gmail.com>
Sent: Monday, May 19, 2025 5:00 AM
To: cp2k
Subject: [CP2K:21474] The total force calculated by CP2K is not zero

Hi all, I’m currently simulating a pure water system with CP2K. Despite configuring appropriate convergence criteria (e.g., CUTOFF, REL_CUTOFF, and EPS_SCF), the total forces from the calculation do not converge to zero. Attached is my input configuration.
Could anyone provide insights into potential causes or recommend literature discussing this specific issue?
Thanks in advance!

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[liu dongfei]

Thanks for your reply!

The attached file is the result of a calculation, showing that the total force of the system is :
-------------------------------------------------------

....
    189      2      H          -0.02270940    -0.01715719    -0.01124088
    190      2      H          -0.00819690     0.01778486    -0.00220956
    191      2      H          -0.00491958    -0.00982567     0.00540266
    192      2      H          -0.00744376     0.00269638    -0.00803075
 SUM OF ATOMIC FORCES           0.00405273    -0.00782982     0.01173920     0.01468126

-------------------------------------------------------

The total force here is not particularly small, and it is comparable to the forces experienced by some H atoms. 

Is this result reasonable, even considering the ripple effect?

best wishes,

Dongfei

[Jürg Hutter]

Hi

These are the total forces for 64 waters from the ripples.
(BTW no k-points here, but why do you have a 3 3 1 grid?)

GPW PBE 0.011
GPW LDA 0.002
GAPW PBE 0.002
GAPW LDA 0.0001

Keep in mind that these forces are correct forces for the energy functional defined in CP2K.

regards
JH

________________________________________
From: cp...@googlegroups.com <cp...@googlegroups.com> on behalf of liu dongfei <liud...@gmail.com>

Sent: Monday, May 19, 2025 4:28 PM
To: cp2k
Subject: Re: [CP2K:21478] The total force calculated by CP2K is not zero

To view this discussion visit https://groups.google.com/d/msgid/cp2k/8c6b7571-cd57-482c-9715-0fb08598e50bn%40googlegroups.com<https://groups.google.com/d/msgid/cp2k/8c6b7571-cd57-482c-9715-0fb08598e50bn%40googlegroups.com?utm_medium=email&utm_source=footer>.

[liu dongfei]

Thank you for your detailed and professional response. I understand now!
This computational result was obtained when I was investigating the performance of different k-point calculation settings. 

In principle, the gamma point calculation setting is reasonable for this system.

best wishes,

Dongfei