Orthogonally Constrained Orbital Optimization: assessing changes of optimal orbitals for orthogonal multi-reference states

TL;DR

This paper introduces an orthogonally constrained orbital optimization method that improves the accuracy of excited state calculations in configuration interaction, outperforming state-average approaches especially in small systems.

Contribution

The paper presents a novel orthogonally constrained orbital optimization technique for accurate state-specific orbitals in multi-reference calculations.

Findings

Accurately recovers excitation energy of a four-electron Hubbard trimer

State-average calculations can significantly underestimate excitation energies

Orbital optimization reduces expansion size and enhances spectroscopic accuracy

Abstract

The choice of molecular orbitals is decisive in configuration interaction calculations. In this letter, a democratic description of the ground and excited states follows an orthogonally constrained orbitals optimization to produce state-specific orbitals. The approach faithfully recovers the excitation energy of afour-electron Hubbard trimer, whereas state-average calculations can miss the value by a factor 2.5. The method emphasises the need for orbitals optimizationto reduce expansions and to reach spectroscopic accuracy.