Application of the nuclear-electronic orbital method to hydrogen transfer systems: Multiple centers and multiconfigurational wavefunctions

The current status of the recently developed nuclear-electronic orbital (NEO) approach is summarized, and the application of this approach to hydrogen tunneling systems is explored. In the NEO approach, specified nuclei are treated quantum mechanically on the same level as the electrons, and mixed nuclear-electronic wavefunctions are calculated variationally with molecular orbital methods. In the application of the NEO approach to hydrogen transfer reactions, the transferring hydrogen nuclei are treated quantum mechanically, and the reaction coordinate depends on only the classical nuclei. For hydrogen tunneling systems, typically the nuclear wavefunction is bilobal and is delocalized between the donor and the acceptor at the transition state. Methodological developments aimed at facilitating the calculation of nuclear-electronic wavefunctions for hydrogen tunneling systems are discussed. These methods include multiple basis function centers, symmetry, and state-averaged NEO-MCSCF (multiconfigurational self-consistent-field). Applications to model proton and hydrogen atom transfer reactions are presented to illustrate the implementation of these methods and to elucidate the fundamental principles of electron-proton correlation in hydrogen tunneling systems.