Time-Dependent and Time-Independent Approaches for the Computation of Absorption Spectra of Uracil Derivatives in Solution

摘要

In this contribution we discuss the most significant benefits and drawbacks of the alternative time-independent (TI) and time-dependent (TD) approaches to the calculation of absorption spectra of molecules in solutions. Eigenstate-free TD methods are in principle the most suitable route to face with the calculation of optical spectra in nonadiabatic systems, most of all in presence of conical intersections easily accessible from the Franck–Condon region. However, here we show that, in selected cases, a combined strategy that adopts TD methods to assess the impact of nonadiabatic couplings on the spectrum and subsequently applies TI methods to include all the degrees of freedom can reveal very convenient. Such a combined TD/TI strategy has been applied to the calculation of the spectrum of Uracil and 5Fluoro-Uracil in acetonitrile. TD studies on reduced dimensionality diabatic models indicate that nonadiabatic effects are moderate and are not the main origin of the diffuse spectral shapes observed in experiments. Subsequent full-coordinate TI calculations allow assigning this feature to intrinsic characteristics of a ππ excitation of a small molecular ring structure. This latter introduces remarkable deformation in all the ring structure thus inducing a FC activity in many molecular normal modes both due to displacements of the equilibrium structures and to Duschinsky mixings of the normal coordinates.