Benchmarking ethylene and ethane: second-order Moller-Plesset pair energies for localized molecular orbitals

The second-order Moller Plesset (MP2) correlation energies of the valence shells Of C2H4 and C2H6 (with fixed nuclear coordinates) were computed as accurately as technically feasible. The pair energies for singlet and triplet pairs are reported for localized molecular orbitals. An accuracy of the order of 0.01 mE(h) is achieved for all of the individual pair energies. The best estimates of the total valence-shell MP2 correlation energies Of C2H4 and C2H6 in the chosen geometries are -373.6+/-0.1 and -410.0+/-0.1 mE(h), respectively. In the aug-cc-pVQZ, aug-cc-pV5Z and aug-cc-pV6Z basis sets, 96.2, 98.0 and 98.8% of the these values are recovered, respectively. The percentages are improved to 99.8% by means of spin-adapted two-point extrapolations from the aug-cc-pVQZ and aug-cc-pV5Z results. An extrapolation based on the aug-cc-pV5Z and aug-cc-pV6Z correlation energies yields 99.9%.