Multireference Methods for Calculating the Dissociation Enthalpy of Tetrahedral P₄ to Two P₂

The potential energy surface for the thermal decomposition of P₄ → 2P₂ was computed along the C_{2 v} reaction trajectory. Single-reference methods were not suitable for describing this complex bond-breaking process, so two multiconfigurational methods, namely, multistate complete active space second-order perturbation theory (MS-CASPT2) and multiconfiguration pair-density functional theory (MC-PDFT), were used with the aim of determining the accuracy and efficiency of these methods for this process. Several active spaces and basis sets were explored. It was found that the multiconfiguration pair-density functional theory method was up to 900 times faster than multistate complete active space second-order perturbation theory while providing similar accuracy.