PyQM/MM: a Python Interface for ONIOM(QM:MM) Calculations with the AMOEBA09 Polarizable Force Field to Study the Chemical Processes in the Interstellar Medium

We have developed a Python interface, PyQM/MM, to perform ONIOM(QM:MM) calculations with the AMOEBA09 polarizable force field. The ONIOM(QM:MM) implementation in PyQM/MM uses the Gaussian16 program for quantum mechanical (QM) computations and the Tinker program for molecular mechanics (MM) computations with the AMOEBA09 polarizable force field. We have used PyQM/MM, employing ONIOM(QM:AMOEBA09) method, to calculate binding energy of SH and OH radicals on amorphous solid water (ASW). Computed binding energies of SH radical are in the range of 0.10-0.36 eV, where the average binding energy is 0.22 eV. Compared to SH radical, OH radical binding energies are stronger (0.21 – 0.52 eV, and the average value is 0.36 eV). We propose that both SH and OH radicals adsorb on ASW, and SH radical binding preference is smaller than OH radical. Also, we have rationalized the mechanism for the reaction between OH anion and CO in ice using ONIOM(wB97X-D:AMOEBA09) method. The computed reaction mechanism showed a relatively low energy path to form HC(O)OH, where the OH anion is recovered during the reaction. In contrast, the reaction between OH radical CO gives rise to HOCO radical. We propose that OH anions and OH radicals in interstellar ices can react with the molecules trapped in ices to synthesize complex organic molecules. These examples evidence that PyQM/MM is a user-friendly strategy to perform ONIOM(wB97XD:AMOEBA09) calculations to study chemical processes in the interstellar medium.