Some recent developments in the simplest-level electron nuclear dynamics method. Theory, code implementation, and applications to chemical dynamics

Electron nuclear dynamics (END) is a time-dependent, variational, direct, and nonadiabatic dynamics method that treats nuclei and electrons simultaneously. While admitting a hierarchy of realizations, the simplest-level END (SLEND), which adopts nuclear classical dynamics and an electronic Thouless single-determinantal state, is the most utilized END version due to its feasibility. SLEND was successfully applied to various gas-phase reactions at intermediate and high energies. However, the SLEND reliance on nuclear classical dynamics alone and/or its lack of dynamical electron correlation inter alia impedes its reliable utilization for more challenging systems. With higher level END realizations overcoming those shortcomings at high computational cost, we advocate solutions that retain SLEND's feasibility due to classical dynamics and the single-determinantal representation. Thus, we advance a novel SLEND/density functional theory (DFT) method wherein electron correlation is included within a feasible single-determinantal representation through DFT procedures. Additionally, we extend a coherent states (CS) quantum reconstruction procedure (CSQRP) to recover some quantum effects from the nuclear classical dynamics; CSQRP now features harmonic, Morse, rotational, and electronic CS. Finally, we improve the SLEND performance by incorporating effective core potentials and implementing our models in our cutting-edge code PACE. The latter features parallel programming and an environment for rapid method development. The new SLEND developments are applied to various gas-phase systems at intermediate and high energies including proton-molecule collisions and Diels-Alder, S_N2, and proton cancer therapy reactions.