software for ligand-receptor interaction energy evaluation ?



Hi,
 a colleague of mine asked me if I could calculate interaction energies between
 a ligand and a receptor into which he docked some compounds.
 I had a look at some of the complexes and it seems like he wants to use this
 for a reevaluation of the activity of his docked structures.
 Unfortunately I am not too much into this sort of energy calculations and now I
 am looking for a freeware program that is able to do that with not too much
 effort.
 I would be very grateful if anybody could point me into a direction.
 Cheers
 Nik
 --
 Nikolaus Stiefl
 Chemometriks and Drug Design
 University Wuerzburg
 Dept. of Pharmacy
 Am Hubland
 D-97074 Wuerzburg
 Germany
 Phone: +49 - 931 8885473