How does LAMMPS perform the calculations

Hello all,

I am new to LAMMPS. I am trying to simulate evaporation of water
molecules in vacuum. I have written a basic script. I want to know how
does LAMMPS calculate E_pair, E_mol, TotEng etc.. I want to do hand
calculations for 3 molecules so that I can verify my results, how can I
do it?
After modeling evaporation of water molecules as single entity I want to
model evaporation of a salt solution of water. i have no idea how to
start with that? Can anyone please give me some start??

Here is how you do it. Read relevant chapters of Frenkel and Smit: