Vikas, I think what you are asking for is something like this, which

gives all 6 components of the pressure:

compute peratom all stress/atom

compute p all reduce sum c_peratom[1] c_peratom[2] c_peratom[3]

c_peratom[4] c_peratom[5] c_peratom[6]

variable press equal -(c_p[1]+c_p[2]+c_p[3])/(3*vol)

variable pxx equal -c_p[1]/vol

variable pyy equal -c_p[2]/vol

variable pzz equal -c_p[3]/vol

variable pxy equal -c_p[4]/vol

variable pxz equal -c_p[5]/vol

variable pyz equal -c_p[6]/vol

thermo_style custom press v_press pxx v_pxx pyy v_pyy pzz v_pzz pxy

v_pxy pxz v_pxz pyz v_pyz

As you can see you must take the negative of stress and divide by

volume to get pressure as described on the doc page

(http://lammps.sandia.gov/doc/compute_stress_atom.html):

"Note that as defined in the formula, per-atom stress is the negative

of the per-atom pressure tensor. It is also really a stress*volume

formulation, meaning the computed quantity is in units of

pressure*volume. It would need to be divided by a per-atom volume to

have units of stress (pressure), but an individual atom's volume is

not well defined or easy to compute in a deformed solid or a liquid.

Thus, if the diagonal components of the per-atom stress tensor are

summed for all atoms in the system and the sum is divided by dV, where

d = dimension and V is the volume of the system, the result should be

-P, where P is the total pressure of the system."

Stan