Periodic Reporting for period 1 - NEOSC (Post Born-Oppenheimer Approximation for Semiclassical Spectroscopy Investigation of Proton-Coupled Electron Transfer Processes)
Periodo di rendicontazione: 2024-01-01 al 2026-01-31
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 project develops an accurate molecular dynamics method implemented as open-source software with two fundamental features. First, a proper treatment of proton-electron coupling to describe chemical processes that break down the Born-Oppenheimer approximation, which is usually taken for granted in chemistry simulations; Second, going beyond the common classical dynamics assumption for the non-hydrogen nuclei. Quantum effects are not confined to H but have also been experimentally observed for C, N, O. The new approach will unify two cutting-edge methods, the Nuclear Electronic Orbital (NEO), which treats selected nuclei (usually hydrogens) beyond the BOA, and the Divide-and-Conquer Semiclassical Initial Value Representation (DC-SCIVR) which accounts for anharmonicity and effectively approximates quantum effects in molecules.
A ubiquitous process in chemistry and biology, involving the quantum-mechanical coupled motion of electrons and protons, is the Proton-Coupled Electron Transfer (PCET) mechanism. The couplings between the transferring protons and the other nuclei' motions are so crucial that the proton transfer does not spontaneously occur without molecular structural relaxation.
PECT is usually investigated experimentally using Time-Resolved InfraRed spectroscopy (TRIR), which follows molecular vibrations in real time. With the new dynamical framework developed in this project, TRIR can be simulated beyond the state of the art, aiding the spectra interpretation and revealing the structural dynamics complementing the proton transfer after photoinduced PCET.
Remarkably, biological photosynthesis catalyzes water splitting through PCET. Therefore, a precise theoretical understanding and accurate simulation of PCET could, in the long term, help make water splitting less energy-intensive and more cost-effective in solar energy devices. PCET is also critical in many fields, such as fuel cells, catalysis, antioxidant reactivity, and chemical synthesis
The new non-BO theory will be implemented in an open-source code, ensuring it will reach the scientific community at large. In fact, TRIR spectroscopy is a widely employed tool, ranging from medical research on DNA damage induced by UV radiation to biological studies of vision-related molecular reactions to the engineering of molecular catalysts for converting solar light into chemical energy.
This project develops an accurate molecular dynamics method implemented as open-source software with two fundamental features. First, a proper treatment of proton-electron coupling to describe chemical processes that break down the Born-Oppenheimer approximation, which is usually taken for granted in chemistry simulations; Second, going beyond the common classical dynamics assumption for the non-hydrogen nuclei. Quantum effects are not confined to H but have also been experimentally observed for C, N, O. The new approach will unify two cutting-edge methods, the Nuclear Electronic Orbital (NEO), which treats selected nuclei (usually hydrogens) beyond the BOA, and the Divide-and-Conquer Semiclassical Initial Value Representation (DC-SCIVR) which accounts for anharmonicity and effectively approximates quantum effects in molecules.
A ubiquitous process in chemistry and biology, involving the quantum-mechanical coupled motion of electrons and protons, is the Proton-Coupled Electron Transfer (PCET) mechanism. The couplings between the transferring protons and the other nuclei' motions are so crucial that the proton transfer does not spontaneously occur without molecular structural relaxation.
PECT is usually investigated experimentally using Time-Resolved InfraRed spectroscopy (TRIR), which follows molecular vibrations in real time. With the new dynamical framework developed in this project, TRIR can be simulated beyond the state of the art, aiding the spectra interpretation and revealing the structural dynamics complementing the proton transfer after photoinduced PCET.
Remarkably, biological photosynthesis catalyzes water splitting through PCET. Therefore, a precise theoretical understanding and accurate simulation of PCET could, in the long term, help make water splitting less energy-intensive and more cost-effective in solar energy devices. PCET is also critical in many fields, such as fuel cells, catalysis, antioxidant reactivity, and chemical synthesis
The new non-BO theory will be implemented in an open-source code, ensuring it will reach the scientific community at large. In fact, TRIR spectroscopy is a widely employed tool, ranging from medical research on DNA damage induced by UV radiation to biological studies of vision-related molecular reactions to the engineering of molecular catalysts for converting solar light into chemical energy.
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.
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.
The project achieved progress beyond the existing state of the art. The NEO-Ehrenfest approach is now integrated with a quasi-classical trajectory (QCT) method, which allows for more precise treatments of anharmonicity and zero-point energy (ZPE) for non-hydrogen nuclei, a capability not previously available. This advancement improved the accuracy of vibrational spectra calculations, particularly for systems like HCN, HNC, and FHF-, and enabled successful benchmarking for more complex molecules, such as H2CO and HCOOH. Furthermore, the new technique and implementation enabled faster simulations of larger systems, opening new possibilities for studying more complex molecular interactions, as evidenced by the successful investigation of the formate-water complex (HCO-·H2O) and its IR spectrum. These results represent a significant improvement in the accuracy and applicability of multi-proton NEO simulations, pushing the boundaries of vibrational spectroscopy.
At present, the implementation of regular IR spectroscopy has been completed, and the next step will extend the NEO-QCT approach to TRIT.
The work so far performed will serve as a basis for the future work in which a semiclassical description of heavy nuclei is eventually achieved, and thus a complete quantum description of the systems. The main challenges include the calculation of Hessian matrix elements needed by semiclassical methods and the interface between NEO codes and semiclassical methods developed in the past (divide-and-conquer and adiabatic-switching semiclassical techniques).
At present, the implementation of regular IR spectroscopy has been completed, and the next step will extend the NEO-QCT approach to TRIT.
The work so far performed will serve as a basis for the future work in which a semiclassical description of heavy nuclei is eventually achieved, and thus a complete quantum description of the systems. The main challenges include the calculation of Hessian matrix elements needed by semiclassical methods and the interface between NEO codes and semiclassical methods developed in the past (divide-and-conquer and adiabatic-switching semiclassical techniques).