pressure at NVT ensemble

Dear Lammps user,
I simulated a box of 200 ionic liquid molecules at NVT ensemble. I wanted achieve the heat capacity at constant pressure using energy fluctuation, and using this this data calculate all the other thermodynamical properties. Everything was fine except that by a little change in volume ( from 6000 angstrom^3 to 13000 angstrom^3 ) the computed pressure changed about 100000 atm ( from 100000 atm to -450 atm ). While via simulating in NVE ensemble ( 398 K ,8500 angstrom^3) I obtained the pressure about 3300 atm for the same system.

1. why the pressure changing is very big?
2. why the pressure is very big ( 6000 angstom^3 and 398 K)?

I used this command:

units real
dimension 3
boundary p p p
atom_style full

bond_style harmonic
angle_style harmonic
dihedral_style opls
improper_style cvff
pair_style lj/cut/coul/long 11
pair_modify mix geometric tail yes
kspace_style pppm 1.0e-4

velocity all create 398 4928459 rot yes dist gaussian

fix 1 all nvt temp 398 398 10.0

timestep 1

Sincerely

Dear Lammps user,
I simulated a box of 200 ionic liquid molecules at NVT ensemble. I wanted
achieve the heat capacity at constant pressure using energy fluctuation, and
using this this data calculate all the other thermodynamical properties.
Everything was fine except that by a little change in volume ( from 6000
angstrom^3 to 13000 angstrom^3 ) the computed pressure changed about

i wouldn't say that more than doubling the volume counts as "a little".

100000 atm ( from 100000 atm to -450 atm ). While via simulating in NVE

liquids are in general not very compressible, thus even *tiny* changes
in volume, let alone changes as massive as yours, will have
significant changes in pressure.

ensemble ( 398 K ,8500 angstrom^3) I obtained the pressure about 3300 atm
for the same system.
1. why the pressure changing is very big?
2. why the pressure is very big ( 6000 angstom^3 and 398 K)?

have you verified that you are simulating a density that is consistent
with the properties of the force field that you are using?

axe;/

Excuse me, but can you tell me whatâ€™s wrong with properties of force field that I used?
thank you

Excuse me, but can you tell me what's wrong with properties of force field
that I used?

i am not clairvoyant. also, i didn't say that anything is wrong with
your force field, all i asked is whether you have verified the force
field that you use and how you know that 6000atm is not the intrinsic
pressure that the force field you use has at the conditions that you
are simulating.

that is a very valid question and any person that does simulations
with classical MD should be able to answer it in a satisfactory
fashion.

axel.