Nuclear-Electronic Orbital Multireference Configuration Interaction for Ground and Excited Vibronic States and Fundamental Insights into Multicomponent Basis Sets.

The nuclear-electronic orbital (NEO) approach incorporates nuclear quantum effects into quantum chemistry calculations by treating specified nuclei quantum mechanically, equivalently to the electrons. Within the NEO framework, excited states are vibronic states representing electronic and nuclear vibrational excitations. The NEO multireference configuration interaction (MRCI) method presented herein provides accurate ground and excited vibronic states. The electronic and nuclear orbitals are optimized with a NEO multiconfigurational self-consistent field (NEO-MCSCF) procedure, thereby including both static and dynamic correlation and allowing the description of double and higher excitations. The accuracy of the NEO-MRCI method is illustrated by computing the ground state protonic densities and excitation energies of the vibronic states for four molecular systems with the hydrogen nucleus treated quantum mechanically. In addition, revised conventional electronic basis sets adapted for quantized nuclei are developed and shown to be essential for achieving this level of accuracy. The NEO-MRCI approach, as well as the strategy for revising electronic basis sets, will play a critical role in multicomponent quantum chemistry.