Nuclear–Electronic Orbital Time-Dependent Configuration Interaction Method

Combining
real-time electronic structure with the nuclear–electronic
orbital (NEO) method has enabled the simulation of complex nonadiabatic
chemical processes. However, accurate descriptions of hydrogen tunneling
and double excitations require multiconfigurational treatments. Herein,
we develop and implement the real-time NEO time-dependent configuration
interaction (NEO-TDCI) approach. Comparison to NEO-full CI calculations
of absorption spectra for a molecular system shows that the NEO-TDCI
approach can accurately capture the tunneling splitting associated
with the electronic ground state as well as vibronic progressions
corresponding to double electron–proton excitations associated
with excited electronic states. Both of these features are absent
from spectra obtained with single reference real-time NEO methods.
Our simulations of hydrogen tunneling dynamics illustrate the oscillation
of the proton density from one side to the other via a delocalized,
bilobal proton wave function. These results indicate that the NEO-TDCI
approach is highly suitable for studying hydrogen tunneling and other
inherently multiconfigurational systems.