Simulation of Ab Initio Optical Absorption Spectrum of β-Carotene with Fully Resolved S₀ and S₂ Vibrational Normal Modes

Abstract

The electronic absorption spectrum of β-carotene (β-Car) is studied using quantum chemistry and quantum dynamics simulations. Vibrational normal modes were computed in optimized geometries of the electronic ground state S 0 and the optically bright excited S 2 state using the time-dependent density functional theory. By expressing the S 2-state normal modes in terms of the ground-state modes, we find that no one-to-one correspondence between the ground- and excited-state vibrational modes exists. Using the ab initio results, we simulated the β-Car absorption spectrum with all 282 vibrational modes in a model solvent at 300 K using the time-dependent Dirac–Frenkel variational principle and are able to qualitatively reproduce the full absorption line shape. By comparing the 282-mode model with the prominent 2-mode model, widely used to interpret carotenoid experiments, we find that the full 282-mode model better describes the high-frequency progression of carotenoid absorption spectra; hence, vibrational modes become highly mixed during the S 0 → S 2 optical excitation. The obtained results suggest that electronic energy dissipation is mediated by numerous vibrational modes.

Modeling molecular J and H aggregates using multiple-Davydov D2 ansatz

“…It is therefore interesting to look at the vibrational mode wavepacket transition from being represented by the D 2 to a more complex mD 2 Ansatz. This transition occurs naturally in eqn (10), for the computation of the linear response function, when an upward transition dipole operator acts on the aggregate ground state, as given by eqn (11). One way to track wavepacket changes is to consider its coordinate and momentum variances, given by…”

“…After independently propagating bra (L) and ket (R) states of eqn (10), their overlap is given by S ð1Þ ðtÞ ¼ C…”

“…It has been widely applied to study excitation relaxation processes in both isolated molecules and molecular aggregates, [3][4][5][6] as well as to compute their linear and nonlinear spectra. [7][8][9][10] While the TDVP method is based on propagating pure wavefunctions, its stochastic extension can be used to describe non-zero temperature by averaging over the initial equilibrium thermal state. 11 However, it still does not properly account for the energy dissipation effect in the vibronic system.…”

“…It has been widely applied to study excitation relaxation processes in both isolated molecules and molecular aggregates, 3–6 as well as to compute their linear and nonlinear spectra. 7–10…”

Inspecting molecular aggregate quadratic vibronic coupling effects using squeezed coherent states

“…It has been extensively used to compute spectra of molecules as well as to examine excitation relaxation dynamics in single molecules and their molecular aggregates. [7][8][9][10][11][12][13][14] The trial wavefunction's selection greatly influences how accurate the method is. It has been shown that in some cases for precise modeling of molecular aggregates, the D 2 ansatz falls short, 15 however, the accuracy of vibrational mode representation can be improved by expanding the available parameter space.…”