Accurate and convergent energetics of color centers by wavefunction theory

TL;DR

This paper introduces a wavefunction theory-based ab initio method for accurately calculating the energetics and properties of color centers in semiconductors, demonstrating its effectiveness on the NV- center in diamond.

Contribution

It develops a wavefunction theory approach combining CASSCF and NEVPT2 for defect energetics, geometry, and properties, offering a robust alternative to DFT for point defects.

Findings

Successfully reproduces the full energy spectrum of NV- center

Quantitatively captures Jahn-Teller effects and fine structure

Analyzes pressure dependence of zero-phonon lines

Abstract

Ab initio description of point defects in semiconductors, characterized by in-gap states of significant multideterminant character, presents a longstanding theoretical challenge for density functional theory (DFT) methods. In this study, we devise a wavefunction theory (WFT) based ab initio methodology as a competing alternative approach. Specifically, we apply perturbation theory (NEVPT2 level) on top of a defect-localized many-body wavefunction (CASSCF level), which provides a balanced description of dynamic and static correlation effects, respectively. This quantum chemical methodology, exemplified for the NV$^-$ center in diamond in this study, is not only used for the calculation of energies and properties, but also for geometry optimization, performed for each electronic state individually. By relaxing cluster models of increasing size and investigating convergence behavior, we quantitatively reproduce (i) the full energy spectrum of NV$^-$ including the recently characterized high-energy states, (ii) the effect of Jahn-Teller distortion on measurable properties, (iii) the fine structure of ground and excited states, (iv) the pressure dependence of zero-phonon lines. Our findings showcase that applying conventional wave-function-based quantum chemistry on carefully crafted clusters can be a robust routine tool for discussing defect-state energetics.