Post Born-Oppenheimer Approximation for Semiclassical Spectroscopy Investigation of Proton-Coupled Electron Transfer Processes (NEOSC)

The Born-Oppenheimer Approximation (BOA) has determined how chemists describe molecules since 1927. BOA separates electronic and nuclear degrees of freedom, considering that the electron timescale is much shorter than the nuclear one. This assumption does not hold in Proton Coupled Electron Transfer (PCET) processes. PCET is the key to efficiency in biological photosynthesis. Time-Resolved Infrared spectroscopy (TRIR) assists PCET investigations but it is usually interpreted with harmonic frequency calculations or classical nuclear dynamics simulations on a BO potential energy surface. To gain further physical insights into PCET it is necessary to revise the theoretical framework for spectroscopy, going beyond the BOA and including a quantum treatment for all nuclei. To reach this goal, this project unifies two cutting-edge methods. One is the Nuclear Electronic Orbital (NEO) approach that goes beyond the BOA by including into the electronic structure calculations the PCET transferred proton. The other is the Semiclassical Initial Value Representation technique that simulates IR spectra by accounting for quantum effects, such as the zero-point energy, overtones, or tunneling, for all nuclei, even in large molecular systems. The project will introduce a new post-BOA conceptual picture for vibrational spectroscopy. Specifically, semiclassical nuclear density calculations will be used to tailor new basis sets to efficiently simulate large molecular systems, gradually including many protons and other nuclei in the NEO wavefunction. In this way, a new spectroscopy theory will be developed for the simulation of TRIR spectra at semiclassical accuracy on NEO post-BOA PESs. These advances will be implemented in an open-source code that we employ to study prototypical PCET systems and gain accurate mechanistic information. This new knowledge will foster applied research by exploiting a deeper understanding of the PCET processes.