Fifth-order many-body perturbation theory for molecular correlation energies

Abstract

Full fifth‐order many‐body perturbation theory (MBPT) correlation energy contributions have been calculated for the molecules: BH, CH₂ , H₂ O, HF, SiH₂ , CO, C₂ H₂, and O₃ at the double zeta plus polarization level. The connected T₄ component of the fifth‐order energy has been computed with only an N⁶ basis set dependence due to a new factorization introduced for the ‘‘connected’’ quadruple T₄ diagrams. For the systems where MBPT is converging smoothly the fifth‐order recovers 30%–50% of the difference between MBPT(4) and the full configuration interaction (FCI) values. The E₅ based [2,1] Padé approximants generally provide a better approximation to the FCI energy. The absolute values of E₅ corrections vary from 1–2 mhartrees for the hydrides at the equilibrium geometry up to 22 mhartrees for the O₃ molecule, which is poorly described by a single self‐consistend‐field reference function.