Natural Orbitals for Wave Function Based Correlated Calculations Using a Plane Wave Basis Set

We demonstrate that natural orbitals allow for reducing the computational cost of wave function based correlated calculations, especially for atoms and molecules in a large box, when a plane wave basis set under periodic boundary conditions is used. The employed natural orbitals are evaluated on the level of second-order Møller–Plesset perturbation theory (MP2), which requires a computational effort that scales as O (N 5), where N is a measure of the system size. Moreover, we find that a simple approximation reducing the scaling to O (N 4) yields orbitals that allow for a similar reduction of the number of virtual orbitals. The MP2 natural orbitals are applied to coupled-cluster singles and doubles (CCSD) as well as full configuration interaction Quantum Monte Carlo calculations of the H2 molecule to test our implementation. Finally, the atomization energies of the LiH molecule and solid are calculated on the level of MP2 and CCSD.