Dear Nikola,
Thank you for the suggestion.
The system is a lutetium oxide cell of 80 atoms, a moderately symmetric
crystal.
You are absolutely right, the initial geometry has the pressure
components of 0.01-0.03 kbar.
Does it make sense to try and optimize it until the pressure is zero?
From this paper I can tell that Nose-Hover approach is a kind of
thermostat, which (apparently) is not implemented in QE for VC-MD.
https://aip.scitation.org/doi/pdf/10.1063/1.463940
I will look into it, but I was hoping for a QE-based solution.
Also, I've found the Wentzcovitch paper to be a very interesting read,
https://journals.aps.org/prb/pdf/10.1103/PhysRevB.44.2358
Here it is stated that Wentzcovitch cell dynamics uses the Lagrangian
from that paper:
https://lists.quantum-espresso.org/pipermail/users/2007-August/007052.html
I've ran some tests with Wentzcovitch cell dynamics to see if it helps.
Apparently, not quite: the cell also oscillates, albeit at a much lower
frequency.
Actually, the Wentzcovitch dynamics (cell_dynamics='w') n_step-volume
curve looks like an oscillating envelope of the Parrinello-Rahman
dynamics (cell_dynamics='pr') n_step-volume curve.
Best regards.
Andrii
Date: Thu, 18 Nov 2021 17:21:10 +0100
From: Nicola Marzari <[email protected]>
To: [email protected]
Subject: Re: [QE-users] Oscillating volume in VC-MD
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
Dear Andrii,
this might be related to the fact that the more harmonic a system is,
the more difficult it is to thermalize it - since it's anharmonicity
that shuffles excitations.
[If you have a system of harmonic oscillators, the populations are good
quantum numbers, i.e. they are constant of motions. A good starting
point is the literature about Nose-hoover chains, disucssing these
difficulties in thermalization.]
In your case, you have low-ish temperatures, and you start from a cell
that has probably a small non-zero pressure, so it oscillates for a
long
time like a pendulum around its equilibrium volume, with very weak
couplings.
nicola
On 18/11/2021 16:45, Andrii Shyichuk via users wrote:
Dear Users,
I am running a VC-MD calculation where the cell volume oscillates in a
nice sinusoidal way, with the period of about 250 fs.
The oscillations are in the range of 99.5-101.5% of the initial volume
for 300K, and within 99-102.5% for 500K.
That happens both with 5 fs (dt=103.353) and 0.5 fs (dt=10.33534) time
steps.
The tolp was 10 in the first case and 2 in the second case.
I use ion_temperature = 'rescaling',? cell_dynamics = 'pr',
ion_dynamics
= 'beeman' and nraise = 1.
I've tried searching the mailing archive and found nothing relevant.
Is it normal? Shouldn't the volume converge to a certain value?
Thank you in advance.
Best regards.
Andrii Shyichuk, University of Wroc?aw
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--
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, SNSF
Head, Laboratory for Materials Simulations, Paul Scherrer Institut
Contact info and websites at http://theossrv1.epfl.ch/Main/Contact
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