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

Publisher Website
Google Scholar
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.