Intermediate vibronic coupling in sexithiophene single crystals

A new approach is proposed to describe intermediate-to-strong linear vibronic coupling in an infinite molecular crystal. The Hamiltonian, transformed to the Lang-Firsov representation, is approximated by disregarding the terms involving more-than-two-particle excitations and block-diagonalized by the Fourier transformation. The spectroscopically relevant block corresponding to zero wave vector is further simplified by introducing a cutoff in the off-diagonal matrix elements and reduced to a manageable size by truncating the basis set, which enables one to diagonalize it numerically. The parametrization, based on independent experiments or theoretical estimates, is aimed to represent the sexithiophene crystal. The results, compared to those obtained for a finite cluster with equivalent material parameters, highlight the favorable convergence properties of the infinite-crystal approach.