Multilevel CC2 and CCSD in reduced orbital spaces: electronic excitations in large molecular systems

TL;DR

This paper introduces efficient multilevel coupled cluster methods (MLCC2 and MLCCSD) combined with a reduced orbital space approach, enabling accurate electronic excitation calculations in large molecular systems with over a thousand atoms.

Contribution

The authors develop a combined multilevel coupled cluster and reduced orbital space methodology, allowing scalable and accurate electronic excitation calculations for large molecular systems.

Findings

Methods successfully applied to paranitroaniline in aqueous solution

Achieved low storage requirements with Cholesky decomposition in truncated basis

Handled systems with more than a thousand atoms efficiently

Abstract

We present efficient implementations of the multilevel CC2 (MLCC2) and multilevel CCSD (MLCCSD) models. As the system size increases, MLCC2 and MLCCSD exhibit the scaling of the lower level coupled cluster model. In order to treat large systems, we combine MLCC2 and MLCCSD with a reduced orbital space approach where the multilevel coupled cluster calculation is performed in a significantly truncated molecular orbital basis. The truncation scheme is based on the selection of an active region of the molecular system and the subsequent construction of localized Hartree-Fock orbitals. These orbitals are used in the multilevel coupled cluster calculation. The electron repulsion integrals are Cholesky decomposed using a screening protocol that guarantees accuracy in the truncated molecular orbital basis. The Cholesky factors are constructed directly in the truncated basis, ensuring low storage requirements. Even larger systems can be treated by using a multilevel Hartree-Fock reference. With the reduced space approach, we can handle systems with more than a thousand atoms. This is demonstrated for paranitroaniline in aqueous solution.