Optimized second‐order correlation orbital manifold for single excitations in the coupled‐cluster method

The sum of the second‐ and fourth‐order Hylleraas functionals and the Newton–Raphson optimization technique have been used to optimize a manifold of first‐ and second‐order correlation orbitals (FOCOS and SOCOS) for the spin‐restricted Hartree–Fock (RHF) wave function. The correlation orbitals are linear combinations of the RHF virtual orbitals. The computational implementation of the new method is discussed and a numerical application to a simple model system is presented to illustrate the capabilities of the proposed methodology.