[CP2K-user] [CP2K:13088] Forces in BOMD calculations

[Lucas Lodeiro]

Hi,
In the case of BO framework, the forces calculated with Hellmann-Feynman
theorem have some instabilities, due to the theorem requires that
<d\phi|\phi> = 0 = <\phi|d\phi>. In most cases this terms are very small,
but requires a very accurate convergence of energy and the wfc to ensure a
completely negligible value.
Then the real forces are equal <dH> + E<d\phi|\phi>.
if <d\phi|\phi> is not negligible, you are avoiding a part of the real
forces and the summation  of them must not be 0.

This in the theory. The implementation of this, uses no infinite numbers
and some noise are in the numerical evaluation, another thing to no cancel
completely.

The important thing with this issue is if you are using and NVE system, the
total energy (kinetic + potential) must be the conserved quantity, not in a
perfect numeral way, but the oscillations must be small and without a
biased direction, if the average (along the fluctuations) total energy is
incrementing or decreasing you have problems in the forces application,
either in the force calculation (more accurate scf cycles) or a too big
time step integrator.

El mié., 8 abr. 2020 a las 13:59, Dmitrii Nikolaev (<
dmitrii.... at gmail.com>) escribió:

> Dear collegues,
>
> I am running the Born-Oppenheimer molecular dynamics, and I notice that
> the sum of the forces on all atoms of my molecule is non-zero. However, as
> I understand for the NVE ensemble the total force should be zero? What is
> the reason for this force?
>
> Thank you in advance,
> Dmitrii
>
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