[QE-users] Oscillating volume in VC-MD

[Andrii Shyichuk]

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 <nicola.marzari at epfl.ch>
> To: users at lists.quantum-espresso.org
> Subject: Re: [QE-users] Oscillating volume in VC-MD
> Message-ID: <93c39f9e-473d-1c31-c0bb-00eb19123c36 at epfl.ch>
> 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
> 
>