CASSCF response equations revisited: a simple and efficient iterative algorithm

TL;DR

This paper introduces a new iterative algorithm for solving CASSCF response equations that simplifies the process and improves efficiency by orthogonalizing trial vectors and reducing the eigenvalue problem size.

Contribution

The paper presents a novel, straightforward iterative method for CASSCF response equations that enhances computational stability and efficiency over existing approaches.

Findings

Algorithm is robust and stable in preliminary tests.

Reduces eigenvalue problem size by half.

Uses orthogonalization with respect to the response matrix scalar product.

Abstract

We present an algorithm to solve the CASSCF linear response equations that is both simple and efficient. The algorithm makes use of the well established symmetric and antisymmetric combinations of trial vectors, but further orthogonalizes them with respect to the scalar product induced by the response matrix. This leads to a standard, symmetric, block eigenvalue problem in the expansion subspace that can be solved by diagonalizing a symmetric, positive definite matrix half the size of the expansion space. Preliminary numerical tests show that the algorithm is robust and stable.