A new intermolecular force field for the liquid-crystal-forming molecule 5CB (4-cyano, 4'n-pentyl biphenyl) has been derived from two-body interaction energies, obtained by the fragmentation reconstruction method (FRM). The accuracy of this purely quantum mechanical approach has been verified by comparing FRM and directly ab initio computed interaction energies, obtaining a very satisfactory agreement with a maximum absolute error lower than 0.4 kcal/mol. The comparison was performed for a large number of internal geometries and dimer arrangements of two fragments of 5CB, namely, 4-cyano biphenyl (OCB) and n-pentyl benzene (5B). The potential energy surface of the 5CB dimer has then been used to parametrize a many-site model interaction potential suitable for computer simulations.