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1. Nuclear–electronic orbital quasiclassical trajectory method for vibrational spectroscopy

   https://doi.org/10.1063/5.0317500  

   Aieta, Chiara; Garner, Scott M; Liu, Aodong; Li, Xiaosong; Hammes-Schiffer, Sharon (April 2026, The Journal of Chemical Physics)

   Simulations of vibrational spectra are important for interpreting experimental data as well as understanding molecular structure and dynamics. Herein, we present an approach for the efficient and accurate incorporation of anharmonicity into such simulations. Real-time nuclear–electronic orbital time-dependent density functional theory treats specified protons quantum mechanically on the same level as the electrons, propagating the electronic and protonic densities according to the time-dependent Schrödinger equation. This approach inherently includes the anharmonicity of the quantum protons and can be combined with Ehrenfest dynamics for the classical nuclei. Herein, this real-time nuclear–electronic orbital (NEO)–Ehrenfest approach is combined with the quasiclassical trajectory (QCT) approach for generating initial conditions that include the zero-point energy of the classical nuclei, thereby enabling sampling of the anharmonic regions of the potential energy surface. The resulting NEO-QCT approach is shown to capture the anharmonic heavy nuclear motion, as well as the anharmonicity of the quantum protons, for a series of molecular systems, including HCN, HNC, FHF−, CH2O, and HCOOH. The NEO-QCT method also captures the distinct spectral features of the formate–water complex (CHO2−⋅ H2O), including the redshifted and broadened OH stretch band due to strong anharmonicity arising from hydrogen bonding and coupling between the motions of the hydrogen nuclei and the heavy nuclei. The NEO-QCT method enables computationally practical simulations of vibrational spectra of molecules that exhibit significant anharmonicity and coupling between vibrational modes. 

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2. Computing excited eigenstates using inexact Lanczos methods and tree tensor network states

   https://doi.org/10.1063/5.0301263  

   Rano, Madhumita; Larsson, Henrik R (October 2025, The Journal of Chemical Physics)

   To understand the dynamics of quantum many-body systems, it is essential to study excited eigenstates. While tensor network states have become a standard tool for computing ground states in computational many-body physics, obtaining accurate excited eigenstates remains a significant challenge. In this work, we develop an approach that combines the inexact Lanczos method, which is designed for efficient computations of excited states, with tree tensor network states (TTNSs). We demonstrate our approach by computing excited vibrational states for three challenging problems: (1) 122 states in two different energy intervals of acetonitrile (12-dimensional), (2) Fermi resonance states of the fluxional Zundel ion (15-dimensional), and (3) selected excited states of the fluxional and very correlated Eigen ion (33-dimensional). The proposed TTNS inexact Lanczos method is directly applicable to other quantum many-body systems. 

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3. Intersystem crossing in tunneling regime: T ₁ → S ₀ relaxation in thiophosgene

   https://doi.org/10.1039/C9CP06956A  

   Lykhin, Aleksandr O.; Varganov, Sergey A. (March 2020, Physical Chemistry Chemical Physics)

   null (Ed.)

   The T 1 excited state relaxation in thiophosgene has attracted much attention as a relatively simple model for the intersystem crossing (ISC) transitions in polyatomic molecules. The very short (20–40 ps) T 1 lifetime predicted in several theoretical studies strongly disagrees with the experimental values (∼20 ns) indicating that the kinetics of T 1 → S 0 ISC is not well understood. We use the nonadiabatic transition state theory (NA-TST) with the Zhu–Nakamura transition probability and the multireference perturbation theory (CASPT2) to show that the T 1 → S 0 ISC occurs in the quantum tunneling regime. We also introduce a new zero-point vibrational energy correction scheme that improves the accuracy of the predicted ISC rate constants at low internal energies. The predicted lifetimes of the T 1 vibrational states are between one and two orders of magnitude larger than the experimental values. This overestimation is attributed to the multidimensional nature of quantum tunneling that facilitates ISC transitions along the non-minimum energy path and is not accounted for in the one-dimensional NA-TST. 

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4. The impact of stacking and phonon environment on energy transfer in organic chromophores: computational insights

   https://doi.org/10.1039/D3TC00479A  

   Mukazhanova, Aliya; Negrin-Yuvero, Hassiel; Freixas, Victor M.; Tretiak, Sergei; Fernandez-Alberti, Sebastian; Sharifzadeh, Sahar (April 2023, Journal of Materials Chemistry C)

   Energy transfer in organic materials is extensively studied due to many applications in optoelectronics. The electronic and vibrational relaxations within molecular assemblies can be influenced by stacking arrangements or additions of a backbone that unites them. Here, we present the computational study of the photoexcitation dynamics of a perylene diimide monomer, and face-to-face stacked dimer and trimer. By using non-adiabatic excited-state molecular dynamics simulations, we show that the non-radiative relaxation is accelerated with the number of stacked molecules. This effect is explained by differences in the energy splitting between states that impacts their corresponding nonadiabatic couplings. Additionally, our analysis of the vibronic dynamics reveals that the passage through the different conical intersections that participate in the relaxation of the stacked systems, activate a positive feedback mechanism. This effect involves a narrow set of vibrational normal modes that accelerate the process by increasing the efficiency of its vibronic dynamics. In contrast, an addition of a biologically inspired backbone slows down the relaxation rate due to its participation in the vibronic dynamics of the molecular stacking arrangements. Our results suggest the stacking arrangements and common backbones as strategies to modulate the efficiency of electronic and vibrational relaxation of diimide-based systems and other molecular aggregates. 

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5. Dynamic signatures of electronically nonadiabatic coupling in sodium hydride: a rigorous test for the symmetric quasi-classical model applied to realistic, ab initio electronic states in the adiabatic representation

   https://doi.org/10.1039/d1cp04090a  

   Talbot, Justin J.; Head-Gordon, Martin; Miller, William H.; Cotton, Stephen J. (February 2022, Physical Chemistry Chemical Physics)

   Sodium hydride (NaH) in the gas phase presents a seemingly simple electronic structure making it a potentially tractable system for the detailed investigation of nonadiabatic molecular dynamics from both computational and experimental standpoints. The single vibrational degree of freedom, as well as the strong nonadiabatic coupling that arises from the excited electronic states taking on considerable ionic character, provides a realistic chemical system to test the accuracy of quasi-classical methods to model population dynamics where the results are directly comparable against quantum mechanical benchmarks. Using a simulated pump–probe type experiment, this work presents computational predictions of population transfer through the avoided crossings of NaH via symmetric quasi-classical Meyer–Miller (SQC/MM), Ehrenfest, and exact quantum dynamics on realistic, ab initio potential energy surfaces. The main driving force for population transfer arises from the ground vibrational level of the D 1 Σ + adiabatic state that is embedded in the manifold of near-dissociation C 1 Σ + vibrational states. When coupled through a sharply localized first-order derivative coupling most of the population transfers between t = 15 and t = 30 fs depending on the initially excited vibronic wavepacket. While quantum mechanical effects are expected due to the reduced mass of NaH, predictions of the population dynamics from both the SQC/MM and Ehrenfest models perform remarkably well against the quantum dynamics benchmark. Additionally, an analysis of the vibronic structure in the nonadiabatically coupled regime is presented using a variational eigensolver methodology. 

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