Cohesive energy density in gromacs?


I would like to calculate cohesive energy density in Gromacs for my (polymer/water) system. I found a formula which get the CED.(The formula is attached here)
But, I dont know exactly what are the parameters.
The article stated that:

where E is the total potential energy in vacuum and in the

condensed state, respectively, and V is the molar volume of material

in its condensed state.

But I don’t know the difference between vacuum and condensed state. Is vacuum state a box with just polymer without water and condensed state a box with water and polymer? how can I get potential energy then? gmx energy? Could you please help me with that?

Thanks a lot

Please can you share the link to the literature?