Optimized virtual ortibal space for high-level correlated calculations

The second order Hylleraas functional and a Newton-Raphson orbital optimization techniquehave been used to generate an active, optimized virtual orbital space (OVOS) of substantially reduced dimension for correlated calculations. Numerical examples for CH₂(¹A₁), C₆H₆, and potential curves for B₂H₆ and H₂O₂ using MBPT and coupled-cluster theory demonstrate that most of the correlation energy can be obtained with a much smaller number of optimized virtual orbitals, and effectively ∼100% of the correlation energy if the OVOS result is combined with the exact second-order energy that is evaluated as a byproduct of the OVOS generation. This suggests a potentially wide applicability of the OVOS method in high accuracy quantum mechanical calculations.