[CP2K-user] [CP2K:18827] Re: Ehrenfest dynamics: Glycine

[Victor Volkov]

Dear Matthew:
good afternoon.
Thank you for the kind reply.

Indeed, the cut-of is too small.
Thank you.

> the timestep is very aggressive.

This is curious.
I always thought that under-sampling (using a longer time-step to speed up
simulated results)
is a problem.
I selected the time-step to over-sample (a bit) a period which would
correspond
to oscillation of a field in near UV - about 230 nm.
Oversampling would be necessary to model circular polarized perturbation
using two fields,
when the second one (0 1 0) is Pi/2 shifted in respect to the first (1 0
0), if "propagation"
is considered along Z (0 0 1).
In principle, I am curious to see two things:
1) if the external field could be such that molecule would break apart;
2) if, then what kind of fragments and how they would fly away according to
the driving conditions.

It is a sort sense if a "destruction of the Hamiltonian" may mimic certain
event upon ionisation,
not all events but some.

Thank you again.
With best regards.
Victor

.




On Mon, May 22, 2023 at 4:56 PM Matt Watkins <mattwatkinsuk at gmail.com>
wrote:

> Hi Victor,
> from your setup the main things are:
>
>    - the cutoff is too small - should be set like a standard DFT
>    calculation - something around 400 Ry and the REL_CUTOFF to 60 Ry or
>    similar.
>    - the timestep is very aggressive. If you plot your energy as function
>    of time I think you will see the conserved quantity drift badly at some
>    point in your simulation and then it explodes. Try something like
>    - stepsize [au_t] 0.1
>
> Matt
>
> On Friday, 19 May 2023 at 07:48:32 UTC+1 Victor Volkov wrote:
>
>> Good day.
>> I am exploring molecular dynamics under field.
>> I set glycine at the center of a box,
>> and suggested Ehrenfest dynamics with a time step of about 0.04 fs,
>> while the custom field file sets the external field to ZERO.
>> The simulation proceeds alright, till the time-frame 500, after which
>> there is a peculiar structural "expansion".
>> *Q1: Would you comment this?*
>> Also, I attach a fragment of the momentum file:
>> CP2K prints reference point position to be "chaotic",
>> while I select REFERENCE COAC: since the atomic charges of the glycine,
>> the reference point should be next to the C-alpha atom.
>> *Q2:  would you comment on the usage of the setting I selected?  *
>> Thank you.
>> Victor
>>
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