Simple Approaches to Calculate Correlation Energy in Polyatomic Molecular Systems

Ab initio calculation including electron correlation are still extremely costly, except for the smallest atoms and molecules. In this paper we present some simple semi-empirical methods to obtain correlation energy. These methods are based on the relation between energy and the off-diagonal density matrix elements, which represent the bonding between atoms in the molecule. The results of our previous studies are reported here and compared with the results obtained by using more accurate techniques.