The project's initial phase involved a comprehensive study of the NEO theory and its implementation using both Q-Chem and the open-source Chronus Quantum software. This foundational work enabled the reproduction of previously published NEO vibrational spectroscopy calculations for simple one-proton quantum systems, specifically focusing on triatomic molecules such as HCN, HNC, and FHF-. These calculations were successfully replicated using existing techniques, providing a solid reference point for future developments.
Building on this, in the following phase, the NEO implementation in Chronus Quantum software was extended to multi-proton systems. This extension was initiated during a scientific visit to the University of Washington (UW) and completed over the next few months through continuous collaboration with the Chronus Quantum development team at UW.
Once the code could handle multiple proton calculations in the already existing NEO methods, an intermediate objective was set: extending the NEO-Ehrenfest approach by integrating a quasi-classical trajectory (QCT) method. This allows for a more accurate treatment of anharmonicity and zero-point energy for non-hydrogen nuclei, which were previously treated at the classical mechanics level.
Further advancements included the identification and development of a strategy to generate initial conditions (velocities) for quasi-classical trajectory simulations in the NEO context, which differs from the conventional Bron-Oppenheimer case, because one has to assign the correct amount of zero-point energy to the classical nuclei portion of the molecule, avoiding double counting of the ZPE of the NEO-quantized protons. As a result, the accuracy of vibrational calculations for previously studied systems such as HCN, HNC, and FHF- was improved, and benchmarks for new systems such as H2CO and HCOOH have been successfully conducted. These results demonstrated the improved precision and capability of the NEO-QCT methodology for simple systems,
A key technical development was the implementation of the NEO-BORT technique, which accelerated simulations of larger systems within the Chronus Quantum framework. This involved designing and integrating test cases and manual entries for the Chronus Quantum code, in close collaboration with the UW and Princeton developers’ teams.
Meanwhile, a set of benchmark cases with available experimental single-molecule infrared spectra has been identified in the literature. Notably, the study of the formate-water complex (HCO-·H2O) revealed interesting quantum effects. Conventional semiclassical calculations showed distinct spectral features in agreement with the experiments, such as a blue shift in the OH stretch band, and significant spectral broadening, suggesting anomalously high anharmonicity and state mixing.
Once the code and scripts were developed and validated, the NEO-QCT approach was employed to calculate the IR spectrum of the formate-water complex, demonstrating its ability to address complex IR spectroscopy scenarios. This successful application led to the preparation of a paper on NEO-QCT, which is currently in press.
During the development phase, two perspective articles about nuclear quantum effects in vibrational spectroscopy have also been published.
