[QE-users] Atomic velocities units

[valeria arteaga muñiz]

Thank you both for your replies.
Okay, I will take that into consideration. Actually I wanted to use the atomic velocities together with the svr thermostat, do you suggest any other method since that one doesn't work well with the atomic velocities?

Best,
Valeria

Valeria

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From: users <users-bounces at lists.quantum-espresso.org> on behalf of Paolo Giannozzi <p.giannozzi at gmail.com>
Sent: Monday, August 2, 2021 8:47:07 AM
To: Quantum ESPRESSO users Forum <users at lists.quantum-espresso.org>
Subject: Re: [QE-users] Atomic velocities units

From a quick look at the code, it seems to me that the documentation is correct for both pw.x and cp.x. For cp.x,  the velocities are given with the same logic as for the atomic positions (in A, Bohr, units of lattice parameter, crystal units) divided by time in hartree a.u. (2.4189*10^-17 s) ; for pw.x, the velocities are always given in Bohr divided by Rydberg a.u. for time (4.8378*10^-17 s). I am rather puzzled by the cp.x logic, but this is how it works right now

Paolo

On Fri, Jul 30, 2021 at 3:07 PM Lorenzo Paulatto <paulatz at gmail.com<mailto:paulatz at gmail.com>> wrote:
I'm not sure, actually the time conversion I gave you is for pw.x, as cp.x use Hartree units instead of Rydberg for the energy there is a possible factor 2 difference (the base to find out this conversion is keep in mind that [ħ]=J·s=1 in atomic units.

The input manual says that pw.x velocities are just "atomic units", no conversion is done in the code as far as I could see.

Also keep in mind that ATOMIC_VELOCITIES is a seldom used feature, and may have bugs (i.e. I recently found out that it does not work with svr thermostat because of a little bug)

hth

--
Lorenzo Paulatto - Paris
On Jul 29 2021, at 8:17 pm, valeria arteaga muñiz <valitzelar at hotmail.com<mailto:valitzelar at hotmail.com>> wrote:

Hello Lorenzo,

Thanks for your reply. This is also for the pw.x code, right?



Best,

Valeria



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Subject: Re: [QE-users] Atomic velocities units


It depends on which units you have specified for atomic positions. Regarding time,
1 a.u.=4.8378 * 10^-17 s


kind regards


--
Lorenzo Paulatto - Paris
On Jul 29 2021, at 5:20 am, valeria arteaga muñiz <valitzelar at hotmail.com<mailto:valitzelar at hotmail.com>> wrote:
  Hello everyone. I am trying to perform a MD run using the pw.x code. I'd like to use the atomic positions and also velocities from another run in Angstroms and Å/picoseconds to the QE input, but I am not sure about the units I need to use. In the documentation, there says it should be atomic units, and when I read the co.c documentation it was the same, but in the description they mentioned we needed to use the units as they're in the atomic positions card and time in a.u. does anyone have an idea of what convertion should I use?
Thanks for your time
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Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
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