Bonding Character as a Descriptor for Huang-Rhys Factors in Optically Active Defects

The electron-phonon coupling of a defect, characterized by its Huang-Rhys (HR) factor, is a crucial metric determining its excited-state dynamics, relevant to defect applications as qubits and quantum emitters. However, HR factors remain challenging to calculate from first principles, complicated by convergence issues in excited-state relaxation and time-consuming phonon calculations. Even when calculated, HR factors lack a rational design principle. Here we show that an orbital-based descriptor can be used to rationalize and efficiently estimate HR factors. Combining this descriptor with a ground-state deformation technique allows circumvention of both excited-state relaxation and full phonon calculations. Specifically, our descriptor for HR factors is constructed using bonding character differences obtained from ground-state density functional theory, quantified by crystal orbital Hamilton populations. We demonstrate this descriptor for prototypical hBN defects and the diamond NV^- center. This orbital-based descriptor can be potentially used in high-throughput computational screening to identify ideal candidates for spin qubits and SPEs.