The UVA response of enolic dibenzoylmethane: beyond the static approach

Sita

J. Chem. Theory Comput.

et al. 2020

We explore the range of applicability of the nuclear ensemble method (NEM) for quantitative simulations of absorption spectra and their temperature variations. We formulate a "good practice" for the NEM based on statistical theory. Special attention is paid to proper treatment of uncertainty estimation including the convergence with the number of samples, which is often neglected in the field. As a testbed, we have selected a well-known chromophore, (E)-azobenzene. We measured its temperature difference UV− vis absorption spectra in methanol, which displayed two dominant features: a moderate increase in the intensity of the nπ* band and a pronounced decrease in intensity of the low-energy part of the ππ* band. We attributed both features to increasing non-Condon effects with temperature. We show that the NEM based on the path integral molecular dynamics combined with range-separated hybrid functionals provides quantitatively accurate spectra and their differences. Experimentally, the depletion of the absorption in the ππ* band showed a characteristic vibrational progression that cannot be reproduced with the NEM. We show that hundreds of thousands of samples are necessary to achieve an accuracy sufficient for the unambiguous explanation of the observed temperature effects. We provide a detailed analysis of the temperature effects on the spectrum based on the harmonic model of the system combined with the NEM. We also rationalize the vibrational structure of the spectrum using the Franck−Condon principle.

Section: Nuclear Ensemble Methodsmentioning

confidence: 98%

Limits of the Nuclear Ensemble Method for Electronic Spectra Simulations: Temperature Dependence of the (E)-Azobenzene Spectrum

Sita

J. Chem. Theory Comput.

et al. 2020

“…RP is a semiclassical approach, which can be derived from both time-dependent and time-independent frameworks. ,, Perhaps the most illuminating is the path-integral formulation, where RP emerges as a zero-order approximation in a semiclassical series . RP imposes neither the Condon approximation nor the harmonic approximation and thus naturally covers the non-Condon effects (the dependence of the transition dipole moment on the nuclear coordinates), symmetrically forbidden transitions, and temperature effects. ,− On the other hand, it ignores quantum interference. , Therefore, it cannot describe vibrational progressions, and the high-energy tail of the spectrum might be underestimated …”

Section: Methodsmentioning

confidence: 99%

Optimal Representation of the Nuclear Ensemble: Application to Electronic Spectroscopy

J. Chem. Theory Comput.

2021

Nuclear densities are frequently represented by an ensemble of nuclear configurations or points in the phase space in various contexts of molecular simulations. The size of the ensemble directly affects the accuracy and computational cost of subsequent calculations of observable quantities. In the present work, we address the question of how many configurations do we need and how to select them most efficiently. We focus on the nuclear ensemble method in the context of electronic spectroscopy, where thousands of sampled configurations are usually needed for sufficiently converged spectra. The proposed representative sampling technique allows for a dramatic reduction of the sample size. By using an exploratory method, we model the density from a large sample in the space of transition properties. The representative subset of nuclear configurations is optimized by minimizing its Kullback–Leibler divergence to the full density with simulated annealing. High-level calculations are then performed only for the selected subset of configurations. We tested the algorithm on electronic absorption spectra of three molecules: (E)-azobenzene, the simplest Criegee intermediate, and hydrated nitrate anion. Typically, dozens of nuclear configurations provided sufficiently accurate spectra. A strongly forbidden transition of the nitrate anion presented the most challenging case due to rare geometries with disproportionately high transition intensities. This problematic case was easily diagnosed within the present approach. We also discuss various exploratory methods and a possible extension to dynamical simulations.

“…10 RP imposes neither the Condon approximation nor the harmonic approximation and thus naturally covers the non-Condon effects (the dependence of the transition dipole moment on the nuclear coordinates), symmetrically forbidden transitions, and temperature effects. 24,[27][28][29] On the other hand, it ignores quantum interference and it cannot describe vibrational progressions. 30,31 Because it is usually difficult to obtain an accurate analytical density, it is more common to use the NEM, i.e., to represent the density with an ensemble of nuclear configurations.…”

Section: Electronic Spectra Within the Nuclear Ensemble Methodsmentioning

confidence: 99%

Optimal Representation of the Nuclear Ensemble: Application to Electronic Spectroscopy

2021

Preprint

Nuclear densities are frequently represented by an ensemble of nuclear configurations or points in the phase space in various contexts of molecular simulations. The size of the ensemble directly affects the accuracy and computational cost of subsequent calculations of observable quantities. In the present work, we address the question of how many configurations do we need and how to select them most efficiently. We focus on the nuclear ensemble method in the context of electronic spectroscopy, where thousands of sampled configurations are usually needed for sufficiently converged spectra. The proposed representative sampling technique allows for a dramatic reduction of the sample size. By using an exploratory method, we model the density from a large sample in the space of transition properties. The representative subset of nuclear configurations is optimized by minimizing its Kullback-Leibler divergence to the full density with simulated annealing. High-level calculations are then performed only for the selected subset of configurations. We tested the algorithm on electronic absorption spectra of three molecules: (E)-azobenzene, the simplest Criegee intermediate, and hydrated nitrate anion. Typically, dozens of nuclear configurations provided sufficiently accurate spectra. A strongly forbidden transition of the nitrate anion presented the most challenging case due to rare geometries with disproportionately high transition intensities. This problematic case was easily diagnosed within the present approach. We also discuss various exploratory methods and a possible extension to dynamical simulations.