Large-Scale Modeling of Proton-Coupled Electron Transfer Based on Absolutely Localized Molecular Orbitals

The calculation of rate constants for proton-coupled electron transfer (PCET) reactions is a challenging task in quantum chemistry. This task involves identifying the mechanism of the process, that can take place either adiabatically or nonadiabatically, and calculating the necessary quantities, such as vibronic couplings, according to the mechanism. Due to different electronic configurations involved, it almost becomes inevitable to use wavefunction-based multireference methods. However, the accurate prediction of rate constants for large molecular systems is limited by the high computational cost of common choices such as complete active space self-consistent field (CASSCF). Since PCET reactions occur in a wide range of biological processes, the development of alternatives with large scalability is of particular interest. A promising alternative is the multistate density-functional theory method based on absolutely-localized molecular orbitals (ALMOs). This gives access to the diabatic donor and acceptor states, adiabatic ground and first excited states, and the electronic coupling. In this work, different operators for the construction of ALMOs are considered. A comparison with CASSCF and N-electron valence state second-order perturbation theory shows that accurate vibronic couplings can be obtained using a non-Hermitian operator. As the method relies on a fragmentation of the system, spectator fragments can be explicitly included with a convenient computational cost. This is demonstrated by the example of a DNA-acrylamide complex.