First-order correlation orbitals for the spin-unrestricted Hartree–Fock zero-order wave function. Electron affinities of C4 clusters

Abstract

The second-order Hylleraas functional and the Newton–Raphson optimization technique have been used to generate first-order correlation orbitals (FOCOs) for the spin-unrestricted Hartree–Fock (UHF) zero-order wave function. The correlation orbitals are linear combinations of the UHF virtual orbitals and are different for electrons with α and β spins. We show that even the number of FOCOs is significantly reduced with respect to the number of all UHF virtual orbitals, the decrease of the second-order correlation energy is rather small. The primary application of the FOCO UHF method is to study larger open-shell molecular systems at higher correlated levels of the theory. The coupled-cluster calculations with FOCOs on the electron affinities of the linear and rhombic carbon C4 clusters, presented here, illustrate the capabilities of the proposed methodology.