NPH and NPT results

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1

Dear LAMMPS users ,

At a two dimensional big system . when I use NPT ensemble simulation on it I can see solid to liquid transition easily but when I change the ensemble to NPH at the same pressure and p damp the instantaneous temperature remains at small amount and system doesn’t have any transition !
This is NPH command that i use :

fix nph all nph iso {pset} {pset} 0.1

also I tried this one :

fix nph all nph iso {pset} {pset} 0.1 couple xyz

Any ideas about what has gone wrong?
Apologize me If I take your time.

Thanks a lot.

Regards,

Saeed.

2

Dear LAMMPS users ,

At a two dimensional big system . when I use NPT ensemble simulation on it I
can see solid to liquid transition easily but when I change the ensemble to
NPH at the same pressure and p damp the instantaneous temperature remains at
small amount and system doesn't have any transition !
This is NPH command that i use :

fix nph all nph iso \{pset\} {pset} 0.1

also I tried this one :

fix nph all nph iso \{pset\} {pset} 0.1 couple xyz

Any ideas about what has gone wrong?

please enlighten us, why you think this is not the expected behavior.

axel.

3

Dear Axel ,

Thanks for your reply.
When I use NPH simulation , instantaneous enthalpy after just two or three step remain at an amount with so small fluctuation . I guess the system doesn’t feel any difference .
I thought maybe it is because of my 2D system size that volume change with NPH simulation doesn’t effect on it !
and when i use NPT simulation on it thermostat play main role for doing simulation and phase transition .

4

Dear Axel ,

Thanks for your reply.
When I use NPH simulation , instantaneous enthalpy after just two or three
step remain at an amount with so small fluctuation . I guess the system
doesn't feel any difference .

i don't understand what you mean by this. science doesn't work by
guessing and simulations even less so. LAMMPS will compute *exactly*
what it is programmed to do. so anything that happens has a specific
reason and can be tracked down. there is no guessing involved.

I thought maybe it is because of my 2D system size that volume change with
NPH simulation doesn't effect on it !

this doesn't make much sense to me either, and it is speculation
without giving any justification.

and when i use NPT simulation on it thermostat play main role for doing
simulation and phase transition .

this is more speculation and doesn't demonstrate any insight into why
you were asking your initial question.

axel.

5

Dear Axel ,

Thanks for your reply .
I think I don’t understand NPH simulation very well so I should study more about that .
Regards,

Saeed.

6

It is not surprising that switching from NPT to NPH has the effect of
suppressing melting. Melting only happens when there is enough energy
in the system. Moreover, even if the system starts out with enough
energy, the melting process absorbs energy (latent heat of melting),
so melting can be arrested, unless there is an outside source of
energy, such as a thermostat.

2015-12-01 18:22 GMT-07:00 SaeeD PourasaD <[email protected]...>:

7

Dear Aidan ,

This means I can’t see melting in my system even if I use high kinetic energy by velocity command from the first step . right ?
What about using high pressure in NPH ? Does it compensate the lack of thermostat ?

8

Temperature is the primary factor, pressure is secondary. If you jack
up the initial energy in an NPH simulation, the crystal will melt.

2015-12-01 20:22 GMT-07:00 SaeeD PourasaD <[email protected]...>: