[gmx-users] Computing the "energy of interaction"

[Golshan Hejazi]

I have a paracetamol crystal and I would like to estimate the paracetamol/paracetamol energy of interaction per unitcell. 

To do this I am doing the following:

Energy of interaction of paracetamols per unitcell= 
Total energy of a unitcell - (Energy of a molecule in the unitcell* number of the molecules in the unitcell)

Energy of interaction of paracetamols per unitcell=  Term1 + Term2


How to compute Term1: 
Took a crystal slab. Compute the total energy(using steepest decent) and divide it by the number of unitcells. The results is: -2443.29 kj/mol

How to compute Term2:
number of the molecules in the unitcell is 4. Energy of a single molecule in vacuum is 490.20 kj/mol 

Therefore:
Energy of interaction of paracetamols per unitcell= -2443.29 kj/mol-(-490.20*4) kj/mol= -482.378 kj/mol


Which is more than a covalent bond! 
Could you please help me to figure out what is going wrong?

Thanks
G.