get 1/3 P in each direction when setting P in x only and zero in y and z

Dear all,

In a metallic system, using NPT I set pressure in x direction and zero pressure in the other two (y and z) directions as follows:
fix ffix all npt temp \{temperature\} {temperature} 0.1 x 100.0 100.0 1.0 y 0.0 0.0 1.0 z 0.0 0.0 1.0

Surprisingly I got average pressure around 33 (bar) in each direction. Is there something wrong in my setting? A complete input file is below:

units metal
boundary p p p
atom_style atomic
read_restart restart.*
pair_style eam/fs
pair_coeff * * \{home\}/code/lammps/lammps\-15Jan11/potentials/Cu\-Zr\_2\.eam\.fs Cu Zr fix fpressure all ave/time 100 10 1000 c\_thermo\_press file pressure\.lmp mode vector ave one thermo 1000 thermo\_style custom step temp pe etotal vol press timestep 0\.001 dump dumptrj all atom 100000 trj\.lammpstrj dump\_modify dumptrj scale no sort id append yes restart 1000000 restart \# write restart file every steps variable temperature equal 1340\.0 fix ffix all npt temp {temperature} ${temperature} 0.1 x 100.0 100.0 1.0 y 0.0 0.0 1.0 z 0.0 0.0 1.0
run 4000000 # temperature increase 0.5 K/ps

Thanks a lot.

Eugene

Try printing pxx pyy pzz with your thermo output.

Steve