The fragment molecular orbital method and understanding monomer polarization

The magnitudes of select π–π interactions are studied with MP2, as well as the fragment molecular orbital (FMO) method, which evaluates the total energy as a sum of polarized monomer energies and interfragment interaction energies. The monomer polarization is of particular interest and is found to increase in magnitude with system size and the electrostatic contribution to the interaction. As a result, differences will exist in the interaction energy reported from FMO calculations, and those calculated using a traditional supermolecular approach.