The study of normal saturated hydrocarbons in the localized representation of the MBPT

An ab initio SCF method with a standard STO-3G basis set was used to study normal saturated hydrocarbons C 2 n +1 H 4 n +4 ( n =0,1,2,3,…). The occupied and the virtual canonical orbitals were localized separately by Boys'procedure and the second-order correlation energy was calculated in the localized representation of the MBPT. In order to separate local and non-local effects the concept of “neighbourhood order” was introduced and the correlation energies were partitioned according to this. It was shown that: (1) by taking into account only the zero-, first- and second-order “neighbourhoods”, more than 99% of the total second-order correlation energy could be recovered; (2) the contributions to the pair correlation energies were transferable to a good approximation; and (3) by increasing the system, the contributions of the most important pair correlation energies converged to definite values, depending only upon the position of the corresponding LMO in the molecule.