Quantum Equation-of-Motion Method with Single, Double, and Triple Excitations

TL;DR

This paper introduces an efficient quantum equation-of-motion method incorporating singles, doubles, and triples excitations, reducing computational cost and improving accuracy for challenging excited state predictions.

Contribution

The authors develop a scalable qEOM method with triple excitations, using symmetry and perturbation theory, and add a correction to improve excitation energy accuracy.

Findings

Achieves less than 0.18 eV error in challenging excited states

Reduces computational scaling from N_o^6N_v^6 to N_o^5N_v^5

Successfully predicts energies for states with large errors in previous methods

Abstract

The quantum equation-of-motion (qEOM) method with singles and doubles has been suggested to study electronically excited states while it fails to predict the excitation energies dominated by double excitations. In this work, we present an efficient implementation of the qEOM method with single, double and triple excitations. In order to reduce the computational complexity, we utilize the point group symmetry and perturbation theory to screen triple excitation operators, and the scaling is reduced from $N_o^6N_v^6$ to $N_o^5N_v^5$. Furthermore, we introduce a perturbation correction to the excitation energy to account for the effect of ignored triple excitation operators. We apply this method to study challenging cases, for which the qEOM-SD method exhibits large errors, such as the 2 $^1\Delta$ excited state of $\rm{CH}^+$ and the 2 $^1\Sigma$ state of $\rm{H}_8$ molecule. Our new method yields the energy errors less than 0.18 eV.