[CP2K-user] [CP2K:13546] Single Molecule Molecular Dynamics

[Lucas Lodeiro]

Thanks Thomas for the advice about the MD.

Yes the geometry optimization was with the same settings, using the default
convergence criteria. I attach the input and output.
There is a little energy difference (1 E-5) between the geo_opt run
(optimized configuration) and the zero step in the MD, due to the centering
positions keyword, I understand that is due to non-continuous fourier grid.
But the potential energy minima along the MD is 1 E-4 lower than the
geo_opt geometry minima... and I cannot understand why.

Regards

El lun., 22 jun. 2020 a las 5:54, Thomas Kühne (<tku... at gmail.com>)
escribió:

> Dear Lucas,
>
> have you conducted the geometry optimization with exactly the same
> settings,
> i.e. basis set, PW cutoff, EPS_SCF, etc.? Since your system has so few
> DOFs,
> fluctuations will be large and conservation by cancellation inherently
> poor.
> Hence, with such a tiny system I would rather stick with NVT instead of
> NVE +
> viral theorem and not use ANGVEL_ZERO …
>
> Greetings,
> Thomas
>
> Am 21.06.2020 um 07:14 schrieb Lucas Lodeiro <eluni... at gmail.com>:
>
> Hi all,
> I want to perform single molecule molecular dynamics.
> First I do a convergence test for cutoff, cell size and poisson solver. I
> read in old mails, that MT is better than WAVELET for MD, due to the first
> is translational invariant.
> After that I start a MD from the optimized molecular geometry at the same
> level of theory. I want to obtain statistical information for a given
> temperature (300K for example), I think an NVT is not well behaved for a
> single molecule, then I ran NVE MD. As the geometry is at the minimum, I
> start with 2*T (where T is the target temperature) thinking in virial
> theorem (at equilibrium equal potential and kinetic energy).
> But the system is not converging (the average temperature) at the desired
> temperature. It shows a weird temperature oscillation (different as
> expected due to bond oscillation), it present in some moment higher
> temperature than the initial and a potential energy lower than the
> initial... Also, the molecular rotation exhibits variations along the MD
> and translation in the box. For that I start a new MD with the same INPUT
> and geometry, adding ANGVEL_ZERO. The MD starts without rotation but in 300
> fs it starts to rotate.
> I attach the files for both MD.
>
> I read about the geometrical constraints, it would be useful for diatomic
> molecules, but for H2O or complexer molecules is not a solution.
>
> Has somebody faced these problems? some advice?
>
> regards
>
>
>
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