Computing HydrogenTunneling Splittings with Nuclear−ElectronicOrbital Multireference Configuration Interaction

Hydrogen tunneling is an important process that impacts reaction rates and molecular spectra. Describing and understanding this process requires a quantum mechanical treatment of the transferring hydrogen. The nuclear−electronic orbital (NEO) approach treats specified nuclei quantum mechanically on the same level as electrons and has recently been implemented at the multireference configuration interaction (MRCI) wave function level. The NEO-MRCI method includes both the static correlation necessary to describe hydrogen tunneling and the electron–proton dynamic correlation required for computing quantitatively accurate nuclear−electronic vibronic states. Herein, the NEO-MRCI method is used to compute the nuclear−electronic wave functions and corresponding vibronic energies for four hydrogen tunneling systems at fixed geometries for a range of donor–acceptor distances. Comparison of the NEO-MRCI results to numerically exact grid-based calculations shows that the NEO-MRCI method can be used to obtain accurate hydrogen and deuterium tunneling splittings at fixed geometries. Thus, this work presents an important component for studying hydrogen tunneling systems.