Pair interaction molecular orbital method: an approximate computational method for molecular interactions

We propose an approximate method for calculating large molecular clusters using molecular orbital (MO) methods. The method allows one to obtain the total energy of a molecular cluster by performing MO calculations on the molecules and the molecular pairs in the system. The computational time is greatly reduced for a system containing a large number of molecules, since the supermolecule calculation of the whole system is avoided. Our method considers important terms of many-body interactions within a pair approximation and is expected to give results of reasonable accuracy even for molecular interactions having significant many-body effects. The numerical calculations of water trimers and tetramers revealed that our method included the major portion of the many-body energies.