# pressure not isotropic in npt

Dear all,

I have a simulation box with about 120*40*40 (x*y*z) angstrom^3 and set npt ensemble via:
fix ffix all npt temp 1300 1300 0.1 iso 0.0 0.0 1.0 # timestep 1 fs

I checked the pressure tensor via command:
fix fpressure all ave/time 100 10 1000 c_thermo_press file pressure.lmp mode vector ave one
and found xx, yy, and zz pressure (when system is in equilibrium) are about +4000, -2500 and -1500 (unit: metal). I was wondering why the pressure is not isotropic, i.e., the same in xx, yy and zz directions (and around zero). Thanks.

Eugene

How big is your system? Is it an isotropic system,
e.g. cubic lattice? Those are not big pressure values
for an atomistic system.

Steve

The system is 120x40x40 angstrom along x-y-z direction. Does iso pressure only applies for cubic system?

Eugene

Quoting Steve Plimpton <[email protected]>:

The system is 120x40x40 angstrom along x-y-z direction. Does iso
pressure only applies for cubic system?

no, but if it _was_ a cubic system,
then the pressure _should_ be isotropic.

the box dimensions themselves have rather little relevance
as to whether the pressure should be isotropic or not, but
what you have _in_ your system matters a lot.

since you don't provide any information, there is not much
else to do than to speculate.

cheers,
axel.

Dear Axel,

The simulation cell contains CuZr alloy, with a fat slab of CuZr crystal in the middle and CuZr liquid filled in the cell. The solid/liquid interfaces are along y-z plane and all atoms are free to move. Thanks.

Eugene

Quoting Axel Kohlmeyer <[email protected]>:

Dear Axel and Steve,

I guess the non-equivalence of x,y,and z pressure components upon iso setting has to do with the initial dimensions of the simulation cell. If, for example, the initial sizes of the cell are such that pressure_xx is positive and pressure_yy and pressure_zz are negative, then the 'dimension coupling' of x,y,z direction due to iso setting makes it difficult for the pressure to be uniform, especially for non-liquid system. In this case aniso setting may help.

Eugene