The effect of velocity variation on temperature

Hi,

I am trying to evaluate the effect of velocity variation on temperature, for a system.

I used the following;

<<<

compute new mobile temp

velocity mobile create 293 564329 temp new

velocity tool set 0.8 0 0 sum yes units box

thermo_style custom step temp

.
.

and l got results below;

Results:

Step Temp

0 188.31046

500 169.23842

1000 167.33972

1500 161.49903

2000 157.51408

2500 105.72243

3000 156.9740

.

.

.

>>>

By varying the velocity vx, l observe the changes to the temperature.
The problem is that l expect the variation to start from around 293K, but it starts from around 188K and sometimes around 165K.
(When l do thermostating for 293K, l don’t encounter problems, but by removing the thermostating and observing the ‘free’ temperature variations, l have the above)

What could be wrong?

Thanks.

Jide

|

Hi,

I am trying to evaluate the effect of velocity variation on temperature, for a system.

jide,

what is the use of this? the (instantaneous) temperature of your system
is uniquely defined by the kinetic energy of the atoms and the number
of degrees of freedom. so you can effectively determine this absolutely;
no need to measure it empirically...

cheers,
axel.

Hi Axel,

Thanks, l agree with you.

I am just wondering while the following statements doesn’t give ‘mobile’ atom group a global temperature of around 293K.

<<<
compute new mobile temp
velocity mobile create 293 564329 temp new
>>>

Or is that not how to make a group of atoms to initiate at a given temp?

Cheers.

Jide

— On Sun, 8/28/11, Axel Kohlmeyer <akohlmey@…12…24…> wrote:

> From: Axel Kohlmeyer [email protected]
> Subject: Re: [lammps-users] The effect of velocity variation on temperature
> To: “Jide Ogunjobi” <jide_ogunjobi@…16…>
> Cc: [email protected]
> Date: Sunday, August 28, 2011, 12:53 AM
>
> On Sat, Aug 27, 2011 at 11:09 AM, Jide Ogunjobi <jide_ogunjobi@…16…> wrote:
> >
> > Hi,
> >
> > I am trying to evaluate the effect of velocity variation on temperature, for a system.
>
> jide,
>
> what is the use of this? the (instantaneous) temperature of your system
> is uniquely defined by the kinetic energy of the atoms and the number
> of degrees of freedom. so you can effectively determine this absolutely;
> no need to measure it empirically…
>
> cheers,
> axel.
>
> >
> > I used the following;
> >
> > <<<
> >
> > compute new mobile temp
> >
> >
> >
> > velocity mobile create 293 564329 temp new
> >
> >
> >
> > velocity tool set 0.8 0 0 sum yes units box
> >
> >
> >
> > thermo_style custom step temp
> >
> > .
> > .
> >
> >
> > and l got results below;
> >
> >
> >
> > Results:
> >
> >
> >
> > Step Temp
> >
> > 0 188.31046
> >
> > 500 169.23842
> >
> > 1000 167.33972
> >
> > 1500 161.49903
> >
> > 2000 157.51408
> >
> > 2500 105.72243
> >
> > 3000 156.9740
> >
> > .
> >
> > .
> >
> > .
> >
> > >>>
> >
> > By varying the velocity vx, l observe the changes to the temperature.
> > The problem is that l expect the variation to start from around 293K, but it starts from around 188K and sometimes around 165K.
> > (When l do thermostating for 293K, l don’t encounter problems, but by removing the thermostating and observing the ‘free’ temperature variations, l have the above)
> >
> > What could be wrong?
> >
> > Thanks.
> >
> > Jide
> >
> >
> >
> >
> > ------------------------------------------------------------------------------
> > EMC VNX: the world’s simplest storage, starting under \$10K
> > The only unified storage solution that offers unified management
> > Up to 160% more powerful than alternatives and 25% more efficient.
> > Guaranteed. http://p.sf.net/sfu/emc-vnx-dev2dev
> > _______________________________________________
> > lammps-users mailing list
> > lammps-users@…459…ourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/lammps-users
> >
>
> –
> Dr. Axel Kohlmeyer
> [email protected] http://goo.gl/1wk0
>
> Institute for Computational Molecular Science
> Temple University, Philadelphia PA, USA.

|

Hi Axel,

Thanks, l agree with you.

I am just wondering while the following statements doesn't give 'mobile' atom group a global temperature of around 293K.

<<<
compute new mobile temp
velocity mobile create 293 564329 temp new
>>>

Or is that not how to make a group of atoms to initiate at a given temp?

yes, but you initialize only the velocities of some atoms,
so unless you define a compute for the temperature of
the group mobile, you won't see the exact same temperature
as a global temperature, since the entire system has more
degrees of freedom.

similarly, you have to consider equilibration when running
with NVE ensemble, i.e. if your initial configuration is close
to the potential energy minimum, then seeding the temperature
to a given temperature, will result in a lower overall temperature,
since some of the initial kinetic energy will be converted into
potential energy. think of a harmonic oscillator: it kinetic energy
is twice as large as the average when you are in the minimum
and zero at the turning points. this is nothing lammps specific,
btw, but rather elementary stat mech stuff.

cheers,
axel.

thermo_style custom step temp

This command will print the T of the
entire system, not the “mobile” atom
temp compute that you defined. See
the doc page for the thermo_style command
for details. You can ask it to print
the T of a specific compute that you have
defined. The compute “temp” is the default
temperature compute for the entire system.

Steve

Dear Axel and Steve,

Thanks for the explanations !

Cheers.

Jide