Optimized virtual orbital space for high-level correlated calculations

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Abstract
The second order Hylleraas functional and a Newton–Raphson orbital optimization technique have 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.