Thermochemical data from ab initio calculations. II. Total correlation energies, Schrödinger energies, and theoretical heats of formation

Bond correlation energies ϵ(XY) have been derived by partitioning second‐order Rayleigh–Schrödinger–Møller–Plesset ( RSMP ) correlation energies. Values of ϵ(XY) depend on the type of bonding between atoms X and Y. They can be considered as comprising correlation energies of bond, lone, and inner‐shell electron pairs of the group XY. Once a set of appropriate increments ϵ(XY) has been obtained, it is possible to estimate unknown RSMP energies of larger molecules. This concept also can be used when estimating total correlation energies E ( CORR ). For this purpose e (XY) values have been derived from known E ( CORR ) energies of small molecules. It is shown that ∣ϵ(XY)∣ increments increase linearly with the number n of electron pairs of the group XY. The function ϵ( n ) becomes zero for n = 1/2 (one uncorrelated electron) and passes through −0.042 for n = 1, which is approximately the correlation energy of the bonding electron pair of H 2 or a 1 s inner‐shell pair. With the aid of estimated E ( CORR ) and HF limit energies, Schrödinger energies and theoretical heats of formation of relatively large molecules are obtained.