Adiabatic semi-empirical parametric method for computing electronic-vibrational spectra of complex molecules .2. Acenes

Calculations of the excited state structure and absorption and emission spectra of acenes (benzene, naphthalene, anthracene and tetracene) have been performed using the devised parametric method. A relatively small structural group H->C= is used as the primary fragment for acenes, as well as for polyenes and diphenylpolyenes. Within the framework of the given method, the simplest ''first approximation model'', which uses only two common parameters for all the groups (delta H-rs(tau)/delta q(i)(b) = 0.08 a.u. and delta(2)H(rs)(tau)/(delta q(i)(b))(2) = 0.3 a.u.), maintains the accuracy of the calculation to approximately 20 cm(-1) for the excited state frequencies and 0.1 for the relative intensities of the vibrational fundamentals. The major peculiarities of the electronic spectra and the fine effects (such as mirror symmetry breakdown) are reproduced in the calculations. Thus quantitative computer simulation of the molecular structural properties, sophisticated treatment of the spectral data and fitting of the model parameters are made possible by the use of this semi-empirical parametric approach, together with the special data bank of molecular parameters. (C) 1997 Elsevier Science B.V.