Dear LAMMPS users,
I am running a very simple system of ellipsoids with 125 ellipsoidal particle, using re-squared potential.
I have used this fix command:
fix 1 all npt/asphere temp 300 300 100 iso 1.00 1.00 100
The unit is real and the timestep is 1 femtosecond.
So I expect the temperature to be 300K, but when I run the system it becomes 600K!
When I set the temperature to be 400K it becomes 800K and also when I set the temperature to be 500K it becomes 1000K!! and so on…
I can not find out what’s wrong with my system. Would you please help me to fix it?
Thank you,
Zahra
The default temp output by the thermo command does
not include rotational degrees of freedom. Make
sure you define such a compute temp/asphere or
use the one that npt/asphere defines and print
its value with thermo output. See the npt/asphere
doc page for the name of its compute.
Steve
Dear Steve,
I added these two lines to my .in file:
compute mytemp all temp/asphere
compute mypress all pressure mytemp
and now the value of temperature reaches my desired value (300K). But now when the temperature reaches 300, the value of pressure starts to fluctuate widely and rapidly! It must have the value 1.0 (atm) and actually at the first steps of my run (before the temperature reaches 300) it has a value between 0.9 to 1.1. But after some more time steps, it starts to fluctuate between -20 to 20 very rapidly!
Do have any suggestion to fix this nonphysical observation?
Thanks
Zahra,
Are you outputting (in the thermo command) the
values of those computes? You won't unless you
request it.
Pressures fluctuate. What evidence do you have
that the fluctuations you are seeing are not correct?
Steve