Simulation of Vibronic Spectra of Flexible Systems: Hybrid DVR-Harmonic Approaches

Abstract

Our general framework for the simulation of vibrational signatures in electronic spectra has been extended to treat one large-amplitude motion (LAM) at the anharmonic level, coupled to the other small amplitude motions (SAM) treated as harmonic. The coupling between LAM and SAM is minimized thanks to the use of delocalized internal coordinates, which are built automatically from the molecular topology. General LAMs can be employed, ranging from intrinsic reaction coordinates to rigid or flexible paths based on the distinguished coordinate approach. The anharmonic model is based on a fully-numerical method based on the discrete variable representation (DVR) theory, supporting different types of boundary conditions. The inclusion of this model in a general-purpose electronic structure code makes available to the user a large panel of quantum chemistry models, for both isolated and condensed phases. The flexibility and reliability of the new framework are illustrated by some case studies, covering various types of LAMs, ranging from a small test case, the photoelectron spectrum of ammonia, to larger systems, such as phenylanthracene and cyclobutanone.

Computational simulation of vibrationally resolved spectra for spin‐forbidden transitions

“…The first one corresponds to a small number of LAMs, which require an ad hoc, anharmonic treatment. The second set of coordinates contains the remaining small‐amplitude modes, which are treated at the harmonic level . From a practical perspective, the reliability of such hybrid schemes depend on the magnitude of the couplings between the two sets of coordinates.…”

“…The second set of coordinates contains the remaining small-amplitude modes, which are treated at the harmonic level. 59 From a practical perspective, the reliability of such hybrid schemes depend on the magnitude of the couplings between the two sets of coordinates. In fact, only by using an appropriate set of coordinates, where those couplings are negligible, can the coordinates belonging to different classes be reliably treated at different levels of theory.…”

High‐Resolution Absorption and Electronic Circular Dichroism Spectra of (R)‐(+)‐1‐Phenylethanol. Confident Interpretation Based on the Synergy between Experiments and Computations

“…The recently reported possibility to introduce anharmonic corrections furtheri mproves the accuracyo ft he simulations, [6] whereas the usage of internal coordinatesa llows remarkable progress to be made also for large molecules characterized by soft modes. [7][8][9][10] In his seminal work, [11] Weigang highlighted that, for weak transitions, the electronic circulard ichroism (ECD) spectra in the region of as ingle electronic state can actually be characterized by bands with different signs. This is an effect of interstate couplings and intensity-borrowing mechanisms.…”

Toward a general mixed quantum/classical method for the calculation of the vibronic ECD of a flexible dye molecule with different stable conformers: Revisiting the case of 2,2,2‐trifluoro‐anthrylethanol

“…In a classical picture, this can be explained realizing that the molecules may have a nonnegligible probability to adopt conformations so distorted to exhibit remarkably different chiroptical properties (e.g., the RS) and vibronic structures with respect to the stable conformers. In principle, accurate spectra for these cases could be obtained considering a realistic model of the anharmonic PES connecting the different conformers and running a full quantum calculation . Yet, this strategy becomes very challenging when more than a few soft modes exist and they are coupled to the remaining vibrational coordinates.…”

“…In principle, accurate spectra for these cases could be obtained considering a realistic model of the anharmonic PES connecting the different conformers and running a full quantum calculation. 24,25 Yet, this strategy becomes very challenging when more than a few soft modes exist and they are coupled to the remaining vibrational coordinates. An alternative route could be provided by classical molecular dynamics (MD), which is very well suited to describe molecular flexibility: if the force field is accurate, MD can provide a realistic distribution of molecular structures, through a collection of snapshots extracted from an equilibrated trajectory.…”