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 Moller-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 H-2 molecule to test our implementation. Finally, the atomization energies of the LiH molecule and solid are calculated on the level of MP2 and CCSD.