Dear Atanu:
Please post replies to the list, not to an individual—this will help you get responses as needed.
To answer your question, LAMMPS does keep track of all six components of the pressure tensor in the variable c_thermo_press. The first three components are p_xx, p_yy, and p_zz, respectively, so you should be able to compute the surface tension as a function of time within the code. (You can do a search for “c_thermo_press” in the LAMMPS manual to find more information on how to use it.)
–AEI