[QE-users] Oscillating volume in VC-MD

[Nicola Marzari]

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 Wrocław
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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