[QE-users] how to obtain random initial velocities with CP

[Eduardo Menendez]

Thank you Riccardo. I followed the link you pointed and I see the issue is
fixed, but I do not know how to download the fixed source file(s).

I have more doubts.

1) I have twice tested the procedure of doing 2 CG steps, then restart the
CP simulation. In both cases  I have gotten an increased electron kinetic
energy, significantly larger than before the CG steps.

2) I have succeded to give initial velocities via random initial positions,
followed by velocity rescaling, but I stil do not get equilibrium. I find
an intriguing statement about the mean square displacements

   Partial temperatures (for each ionic specie)
   Species  Temp (K)   Mean Square Displacement (a.u.)
        1   5.54E+02     1.9353E+03
        2   3.12E+02     1.9354E+03
        3   2.52E+01     1.9352E+03
        4   5.43E+02     1.9354E+03
        5   1.03E+02     1.9355E+03

3) finally, regarding the documentation statement:

'ATOMIC_VELOCITIES'
 BEWARE: works only if restart_mode='from_scratch',
 tested only with electrons_dynamics='cg'

I guess one cannot use 'sd' or 'cp', unless it admits
restart_mode='restart', otherwise the wfc would be random. Am I missing
something?

Best regards,

Eduardo Menendez Proupin


---------- Forwarded message ----------
From: Riccardo Bertossa <rbertoss at sissa.it>
To: users at lists.quantum-espresso.org
Cc:
Bcc:
Date: Tue, 23 Oct 2018 10:53:28 +0200
Subject: Re: [QE-users] how to obtain random initial velocities with CP

Hi,

I had too the issue with starting random velocities.
I think I've fixed it in the merge request
https://gitlab.com/QEF/q-e/merge_requests/189, can you please give it
a try?


Regarding the reading of the velocities from the input:
the documentation was wrong, because the units of the velocities were
not correctly stated, it was corrected few months ago. I think that it
works also without CG.
But please note that after manually setting the velocities you should
do a step of CG, that in fact does two steps in this case
(one with the starting ions position and one with the position updated
with the new velocities), to set the initial wavefunction velocity to
a reasonable value with respect to the ions velocities.
In my experience, if you don't do this, the kinetic energy that the
wfc gain at the end is bigger than the kinetic energy that you get
with the electron velocities calculated in the "correct" way.

Note also that in the new version, I added an option "change_step"
also for the electron_velocities when you change step. This was a
little modification to allow a smoother change
(before the electron velocities was not updated if the timestep
changed, increasing the kinetic energy of the electrons that were no
more moving, in some sense, following the ions).

electron_velocities CHARACTER

'zero'      : restart setting electronic velocities to zero
'default'   : restart using electronic velocities of the
            previous run
'change_step' : restart simulation using electronic velocities of the
            previous run, with rescaling due to the timestep change.
            specify the old step via tolp as in
            tolp = 'old_time_step_value' in au.
            Note that you may want to specify
             ion_velocities = 'change_step'


Note also that the change of dt is correctly implemented in the autopilot module
(so, it is not necessary to use a restart file, and you can change
easily dt on the fly and try more values of it)


Best regards



Riccardo Bertossa
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