FCclasses3: Vibrationally‐resolved spectra simulated at the edge of the harmonic approximation

Abstract

We introduce FCclasses3, a code to carry out vibronic simulations of electronic spectra and nonradiative rates, based on the harmonic approximation. Key new features are: implementation of the full family of vertical and adiabatic harmonic models, vibrational analysis in curvilinear coordinates, extension to several electronic spectroscopies and implementation of time-dependent approaches. The use of curvilinear valence internal coordinates allows the adoption of quadratic model potential energy surfaces (PES) of the initial and final states expanded at arbitrary configurations.Moreover, the implementation of suitable projectors provides a robust framework for defining reduced-dimensionality models by sorting flexible coordinates out of the harmonic subset, so that they can then be treated at anharmonic level, or with mixed quantum classical approaches. A set of tools to facilitate input preparation and output analysis is also provided. We show the program at work in the simulation of different spectra (one and two-photon absorption, emission and resonance Raman) and internal conversion rate of a typical rigid molecule, anthracene. Then, we focus on absorption and emission spectra of a series of flexible polyphenyl molecules, highlighting the relevance of some of the newly implemented features. The code is freely available at http://www.iccom.cnr.it/en/fcclasses/.

In Silico Investigation of the Photoisomerization Mechanism of Push-Push Azoderivatives

“…Moreover, a vibrationally-resolved version of the absorption spectra was calculated in vacuum at the B3LYP/cc-pVDZ level of theory, using the code FCClasses3 developed by Santoro et al [61,62]. The vibrationally resolved absorption spectra for the cis isomers were also calculated at the B3LYP/cc-pVDZ level.…”

Unraveling the contributions to the spectral shape of flexible dyes in solution: insights on the absorption spectrum of an oxyluciferin analogue

“…When the adiabatic PESs are described at the harmonic approximation, analytical expressions for both TI and TD formulations of the vibronic spectrum have been derived, and are implemented in our code FC classes3. 48,49 TD approaches are especially efficient in getting fully converged spectra at arbitrary temperatures, hence we adopt such formulation in all calculations conducted in this work. The harmonic model implies a quadratic expansion of the initial and final PESs, which is performed at the minimum for the initial state (the so-called Franck-Condon point), and either at the minimum (adiabatic models) or the Franck-Condon geometry (vertical models) for the final state.…”

Vibrationally resolved two-photon electronic spectra including vibrational pre-excitation: Theory and application to VIPER spectroscopy with two-photon excitation

“…We can now use Feynman's path integral expression to evaluate the matrix elements of the excitedstate propagator Eq. (19),…”

“…Vibrationally resolved absorption spectra using 2P excitation were obtained using the FC-classes3 code. 18,19 Analogously to the 1P case, all spectra using the time-independent formalism were obtained with N max = 10 8 and subsequently convoluted with a Lorentzian lineshape with a Half Width at Half Maximum (HWHM) of 0.1 eV. Complementary time-dependent vibrationally resolved spectra were obtained through the analytical correlation function approach (see Sec.…”

“…To this end a time-independent version of the FCclasses method, 17,18 permitting the efficient evaluation of Franck-Condon overlap integrals, is employed. In the present context, the recent FC-Classes3 version 18,19 is used, which is specifically adapted to include 1P and 2P absorption in the presence of vibrational pre-excitation. Complementary to the time-independent (TI) FCclasses approach, a time-dependent (TD) approach is employed which relies on the computation of correlation functions by the analytical evaluation of Gaussian moments, within a harmonic approximation including Duschinsky effects.…”