stress and pressure.

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

"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."