Some general comments:
I hope you saw Stan Moore’s post on your other thread. Your current input means that the group left_drop contains molecules 1 AND 1747. Only those two molecules.
To select a molecule range you need to specify 1:1747. Note the colon.
I am not sure what you want to do with your potential energy calculation. MD potential energies aren’t particularly useful unless measured relative to some sensible reference configuration. (As a simple example – the energy of “the water drop on the surface” includes both the water-carbon adhesion and the water-water deformation, since presumably some surface water molecules will be geometrically frustrated relative to their ideal bulk solvation environment.)
One eV is 23.06 kcal/mol. The mathematical inverse operation is not controversial.