Orbital Optimization in the Active Space Decomposition Model

TL;DR

This paper develops and implements orbital optimization algorithms for the active space decomposition (ASD) model, extending CASSCF methods to better analyze electron and exciton dynamics in complex molecular systems.

Contribution

It introduces orbital rotation techniques within ASD, enabling unambiguous active space partitioning and application to electron and exciton transfer processes.

Findings

Successful application to chromophore systems

Analysis of triplet energy transfer mechanisms

Model Hamiltonians for charge transfer processes

Abstract

We report the derivation and implementation of orbital optimization algorithms for the active space decomposition (ASD) model, which are extensions of complete active space self-consistent field (CASSCF) and its occupation-restricted variants in the conventional multiconfiguration electronic-structure theory. Orbital rotations between active subspaces are included in the optimization, which allows us to unambiguously partition the active space into subspaces, enabling application of ASD to electron and exciton dynamics in covalently linked chromophores. One- and two-particle reduced density matrices, which are required for evaluation of orbital gradient and approximate Hessian elements, are computed from the intermediate tensors in the ASD energy evaluation. Numerical results on 4-(2-naphthylmethyl)-benzaldehyde and [3$_6$]cyclophane and model Hamiltonian analyses of triplet energy transfer processes in the Closs systems are presented. Furthermore model Hamiltonians for hole and electron transfer processes in anti-[2.2](1,4)pentacenophane are studied using an occupation-restricted variant.