Particle-particle random phase approximation for predicting correlated excited states of point defects

We present a particle-particle random phase approximation (ppRPA) approach that accurately predicts correlated excited states of point defects in solids, such as nitrogen-vacancy and carbon-vacancy centers in diamond as well as silicon-vacancy and divacancy centers in silicon carbide. Starting from the (N+2)/(N-2)-electron ground state calculated with the density functional theory (DFT), the ppRPA excitation energies of the N-electron system are computed as the differences between the two-electron removal/addition energies of the (N+2)/(N-2)-electron system. We demonstrate ppRPA within an active space scheme has lower (or comparable) computational cost than TDDFT, while treating multiference ground and excited states more reliably and having weak DFT starting-point dependence. Particularly, ppRPA with the B3LYP functional yields accurate excitation energies with errors smaller than 0.2 eV for tested systems compared to available experimental values.