Accurate diabatic potential energy model for NO3 including spin–orbit coupling

Spin-orbit (SO) coupling effects can play an important role in the accurate theoretical treatment of the quantum dynamics and the spectroscopy of molecular systems. It often goes hand in hand with vibronic coupling and thus calls for a diabatic treatment of all relevant couplings. While the treatment of vibronic coupling is well established, accurate diabatic SO models remain scarce. Therefore, a detailed study of the SO coupling for the relevant electronic states of the nitrate radical (NO3) is presented here. In contrast to most of the previously studied systems, the SO coupling effect is distributed across several atoms and cannot be localized at a single relativistic atom. Based on our previously developed diabatic potential energy model for NO3, a fully geometry-dependent diabatic SO and vibronic coupling model is presented. A link to the atomic nature of SO coupling is established based on an intuitive and simple semi-quantitative model, and the geometry dependence of the SO coupling is analyzed in detail. The necessity of including the geometry dependence is carefully analyzed for the different kinds of nuclear motions. Experimental evidence for SO coupling effects in NO3 has been reported previously, and the present results are in excellent agreement with these experimental findings.