Nonorthogonal Active Space Decomposition of Wave Functions with Multiple Correlation Mechanisms

Abstract

The nonorthogonal active space decomposition (NO-ASD) methodology is proposed for describing systems containing multiple correlation mechanisms. NO-ASD partitions the wave function by a correlation mechanism, such that the interactions between different correlation mechanisms are treated with an effective Hamiltonian approach, while interactions between correlated orbitals in the same correlation mechanism are treated explicitly. As a result, the determinant expansion scales polynomially with the number of correlation mechanisms rather than exponentially, which significantly reduces the factorial scaling associated with the size of the correlated orbital space. Despite the nonorthogonal framework of NO-ASD, the approach can take advantage of computational efficient matrix element evaluation when performing nonorthogonal coupling of orthogonal determinant expansions. In this work, we introduce and examine the NO-ASD approach in comparison to complete active space methods to establish how the NO-ASD approach reduces the problem dimensionality and the extent to which it affects the amount of correlation energy recovered. Calculations are performed on ozone, nickel–acetylene, and isomers of μ-oxo dicopper ammonia.

Nonorthogonal Multireference Wave Function Description of Triplet–Triplet Energy Transfer Couplings

“…As a result, a more complete theory can be formulated through a determinantal expansion of each quasi-diabatic SCF solution. While there are a number of ways to achieve such an expansion, including UNOCAS, half-projection − (or a spin coupled expansion in the case of more than two single-electron sites), or projection through collective rotation of the electron spin axis, , in the case of TEnT, the diabatic states are all single determinant in nature owing to their high-spin character. Therefore, the use of quasi-diabatic SCF solutions is particularly suitable for determining diabatic electronic coupling in TEnT, especially as they automatically recover electron relaxation in response to the change in electron configuration.…”

Non-orthogonal Configuration Interaction Study on the Effect of Thermal Distortions on the Singlet Fission Process in Photoexcited Pure and B,N-Doped Pentacene Crystals

“…In this way, the final wave function expansion remains short yet accurate, facilitating the interpretation of the physics of the system. This facility for interpretation, combined with the increased computer power, is one of the main reasons why NOCI has regained attention over the last decade and various slightly different implementations and applications have been recently reported. − For the NOCI calculations, we applied the massively parallel and GPU-accelerated GronOR program, which provides coupling parameters obtained from the pair states with and without dynamical electron correlation corrections . However, only the results with dynamical electron correlation corrections are herein reported.…”