Calculation of potential of single water molecule and its rotation energy

In my case, the protein aqueous solution was simulated with SPC/E model by GROMACS.

  1. I wonder how the potential of single water molecule is calculted during the simulation.Has the interactions between any atom(O,H1,H2) of the target water molecule and all of other atoms(belonging to protein and water molecules) been taken into account when calcuting the L-J potential of this target water molecule?
  2. How to calculte the rotation energy for every single water molecule?
    Any suggestions will be appreciated. Thanks very much in advance!