Abstract
We present a method to approximate post-Hartree-Fock correlation energies by using approximate natural orbitals obtained by the random phase approximation (RPA). We demonstrate the method by applying it to the helium atom, the hydrogen and fluorine molecule, and to diamond as an example of a periodic system. For these benchmark systems, we show that RPA natural orbitals converge the MP2 correlation energy rapidly. Additionally, we calculated full configuration interaction energies for He and H2, which are in excellent agreement with the literature and experimental values. We conclude that the proposed method may serve as a compromise to reach good approximations to correlation energies at moderate computational cost, and we expect the method to be especially useful for theoretical studies on surface chemistry by providing an efficient basis to correlated wave function based methods.
Highlights
In ab initio quantum chemistry and computational materials physics, there exists a well-known trade-off between accuracy and computational cost
Assuming that with only a few random phase approximation (RPA) natural orbitals one can span the relevant subspace of the one electron wave functions for advanced methods like MP2 or configuration interaction (CI), we aim to accurately approximate electronic ground state energies of these methods using a small number of virtual states
The calculation of RPA natural orbitals (RPANOs) scales only cubically with system size. This favorable scaling affords the proposed method an advantage over a similar method using MP2 natural orbitals, which scale with the fifth order of the system size in the canonical implementation
Summary
In ab initio quantum chemistry and computational materials physics, there exists a well-known trade-off between accuracy and computational cost. While mean field methods like Hartree-Fock (HF) and density functional theory (DFT) possess computationally favorable scaling of N3 with the number of electrons, they sometimes lack accuracy and fail to describe certain processes. Describing the dissociation of simple molecules like H2 already poses a challenge for such methods.. The properties that are not well described by the HF approximation are attributed to the so-called electron correlation, and there exists a wide variety of electronic structure methods all of which attempt to take the correlation effects into account in an approximate manner. Most of them have in common that one can increase the accuracy systematically at the expense of computational resources, e.g., Møller-Plesset perturbation theory (MPn), coupled cluster (CC), and configuration interaction (CI) methods
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