A New Potential Energy Surface of the PO⁺-H₂ Complex and Intermolecular Rovibrational State Calculations

The recent detection of the phosphorus monoxide cation (PO⁺) in the interstellar medium (ISM) has generated considerable interest in its collisional excitation and reactivity in such environments. Due to the difficulties in conducting laboratory experiments in these extreme environments, theoretical calculations have become essential to model the excitation and reactivity of PO⁺. In this context, several theoretical studies have been conducted to better understand its abundance and impact on interstellar chemical processes. An important aspect of these studies is the accurate characterization of their electronic interaction with the surrounding gas constituents. We present here a new four-dimensional potential energy surface (PES) for the interaction between the PO⁺ cation and the H₂ molecule, the dominant species in the cold ISM, using the explicitly correlated coupled cluster method with single, double, and perturbative triple excitations [CCSD(T)-F12a]. The rigid rotor PES provides a global representation of the PO⁺-H₂ interaction, and presents a unique global minimum with a well depth of 1252.88 cm^-1. We subsequently characterized the rovibrational states of the PO⁺-H₂ complex, up to a total angular momentum J of 3, by solving the nuclear Schrödinger equation with the block-induced relaxation procedure implemented in the Heidelberg Multi-Configuration Time Dependent Hartree (MCTDH) package. We obtained zero-point energies of 422.201 cm^-1 for PO⁺-para-H₂ and 487.805 cm^-1 for the PO⁺-ortho-H₂ complex. This corresponds to dissociation energies (D₀) of 830.679 and 765.075 cm^-1for PO⁺-para-H₂ and of 487.805 cm^-1 for the PO⁺-ortho-H₂ complex. We hope that the present theoretical results will stimulate experimental studies of the PO⁺-H₂ complex in order to validate the predictions reported in this work.