An Efficient Algorithm for Constrained CASSCF(1,2) and CASSCF(3,2) Simulations as Relevant to Electron and Hole Transfer Problems

TL;DR

This paper introduces an efficient algorithm for constrained CASSCF simulations, significantly reducing computational costs and enabling faster non-adiabatic dynamics studies of charge transfer states.

Contribution

The paper presents a novel algorithm that separates the constrained minimization into SCF and nSCF problems and accelerates the SCF step, achieving substantial speedups over previous methods.

Findings

Computational cost reduced by a factor of 8 to 20.

Algorithm applicable to other constrained minimization problems.

Enables faster non-adiabatic dynamics simulations once gradients are available.

Abstract

We propose an efficient algorithm for the recently published electron/hole-transfer Dynamical-weighted State-averaged Constrained CASSCF (eDSC/hDSC) method studying charge transfer states and D$_1$-D$_0$ crossings for systems with odd numbers of electrons. By separating the constrained minimization problem into an unconstrained self-consistent-field (SCF) problem and a constrained non-self-consistent-field (nSCF) problem, and accelerating the direct inversion in the iterative subspace (DIIS) technique to solve the SCF problem, the overall computational cost is reduced by a factor of 8 to 20 compared with directly using sequential quadratic programming (SQP). This approach should be applicable for other constrained minimization problems and, in the immediate future, once gradients are available, the present eDSC/hDSC algorithm should allow for speedy non-adiabatic dynamics simulations.