[lammps-users] Question about NPT

Dear Sir,

I am a little new in the forum and in the domain.

I am currently doing some simple simulations using a periodic water SPC model that I programmed in the Lammps.
I have done some NVE calculations and some NPT calculations.
I am currently observing that the simulation time is greater (about 15%) for the NPT calculation than for the NVE calculation.
In particular, the simulation time between two thermo outputs is not constant in the case of a NPT simulation.

My question is the following.
(1) For NPT simulations, the Volume is a variable. The position of the atoms change with the Volume.
A fix shake is carried out in order not to take into account the intramolecular interactions (due to the SPC model) and it may be the
reason why the simulation time is greater. Is this correct?
(2) In the case where it is correct, is it possible for the Lammps
-- to change the center of mass of the water molecule and do a global translation as the Volume varies
-- and then do the fix shake
and how do we proceed?

I thank you for your help

Evangeline Capiez-Lernout

Fix shake and NPT are more expensive than NVE with no shake.
15% is reasonable.

LAMMPS doesn't know or compute the c-o-m of each water
molecule, so it doesn't move the c-o-m when the volume
dilates. It just moves the atoms, then SHAKE will fix it.
This is correct behavior.