# [Pw_forum] Temperature extraction from PW

Zahraa Ibrahim Zahraa.Ibrahim at uoit.ca
Sat Sep 1 04:52:23 CEST 2007

```Hello Antonio

The center of mass moves slightly along a given direction. So it is most probably not fixed, does the 3 maybe represent the three rotational degrees of freedom, which due to the translational symmetry of the cell can not be excited?

Zahraa Ibrahim
UOIT

-----Original Message-----
From:	pw_forum-bounces at pwscf.org on behalf of Antonio Tilocca
Sent:	Fri 8/31/2007 7:10 AM
To:	pw_forum at pwscf.org
Cc:
Subject:	Re: [Pw_forum] Temperature extraction from PW

I think the extra 3 takes into account the translational (center of mass)
degrees of freedom: if the center of mass is fixed, the total number of
degrees of freedom decreases by 3.
At the same time, the c.o.m. kinetic energy will be
zero, so there is probably no need to subtract it from the total kinetic
energy in the numerator.

Antonio Tilocca
UCL

Message: 5
Date: Thu, 30 Aug 2007 13:21:11 -0400
From: "Zahraa Ibrahim" <Zahraa.Ibrahim at uoit.ca>
Subject: [Pw_forum] Temperature extraction from PW

I noticed that the temperature in the espresso version is calculated from
T = 2 * (kinetic energy - center of mass energy) / (3N-3) [I am not
setting any
constraints] while in the democritus version it was calculated by dividing
the
numerator by (3N).  (N being the number of particles).
I was wondering what exactly this extra 3 stands for.

Another, probably related, question is about the effect of periodic
boundary
conditions on the temperature. I am simulating thermal motion in a solid
with a
relatively small number of atoms. Do the periodic boundary conditions
change
the degrees of freedom of the solid in any ways, that need to be taken
into
account in the temperature estimation.

Greetings
Zahraa Ibrahim

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