# Boundary Conditions for Twisting simulation.

Hi Vikas,

I’m just wondering why you cant include the atoms in the ‘fix move rotate’ in the thermostat?
Does this also mean that the atoms in this group should not be included in an ensemble?
I suppose my understanding of the difference between an ensemble and a thermostat is important here? (Im guessing that only those ensembles that fix temperature are thermostats)
Sorry if these questions are very fundamental.

Daniel

Hi Vikas,

I'm just wondering why you cant include the atoms in the 'fix move rotate'
in the thermostat?

technically you can, but it would give you unphysical (=wrong) results.

Does this also mean that the atoms in this group should not be included in
an ensemble?

this question makes no sense. your (total) system *always* forms an
ensemble. whether this is a well defined statistical mechanical
ensemble or not is a different question. the use of fix move will
automatically result in an non-standard ensemble.

I suppose my understanding of the difference between an ensemble and a
thermostat is important here? (Im guessing that only those ensembles that
fix temperature are thermostats)

*never* guess. what is an ensemble is explained in great detail in
text books on statistical mechanics. you can also how this can be
influenced by a thermostat.

now in LAMMPS there are different kinds of fixes that are relevant to
these subjects:
- fixes that just change things based on a given recipe like fix move
or fix deform
- fixes that just do time integration (fix nve)
- fixes that work only as a thermostat or barostat (fix langevin, fix
temp/berendsen, fix temp/rescale, fix press/berendsen)
- fixes that do time integration and thermostat/barostat (fix nvt, fix npt, ...)

fixes like fix nve, nvt, npt, etc. have their name, because you would
get the corresponding ensemble if you have a fully periodic system
*and* no other manipulation of the system. so it is not the fix that
determines the ensemble but it is a prerequisite for it.
also, it should be self-evident that you must not have atoms being
moved by two kinds of fixes and not have atoms that are moved by a
different fix included in the temperature compute of a thermostat.
this is usually taken care of automatically, but your global
temperature display is using all atoms, and that is not likely to be
meaningful to you, if you use fix move on some of them.

Sorry if these questions are very fundamental.

there are two parts to that. the lammps specific part and the
statistical mechanics specific part. the second part you'll have to
teach yourself, this is not the right forum for it.

axel.

Dear Daniel,
I am sure Axel answered all your questions but better than I could

Regards,
Vikas