Particle–Particle Random Phase Approximation for Predicting Correlated Excited States of Point Defects

Abstract

The particle–particle random phase approximation (ppRPA) within the hole–hole channel was recently proposed as an efficient tool for computing excitation energies of point defects in solids [J. Phys. Chem. Lett. 2024, 15, 2757–2764]. In this work, we investigate the application of ppRPA within the particle–particle channel for predicting correlated excited states of point defects, including the carbon-vacancy (VC) in diamond, the oxygen-vacancy (VO) in magnesium oxide (MgO), and the carbon dimer defect (C_BC_N) in two-dimensional hexagonal boron nitride (h-BN). Starting from a density functional theory calculation of the (N – 2)-electron ground state, vertical excitation energies of the N-electron system are obtained as the differences between the two-electron addition energies. We show that active-space ppRPA with the B3LYP functional yields accurate excitation energies, with errors mostly smaller than 0.1 eV for tested systems compared to available experimental values. We further develop a natural transition orbital scheme within ppRPA, which provides insights into the multireference character of defect states. This study, together with our previous work, establishes ppRPA as a low-cost and accurate method for investigating excited-state properties of point defect systems.