IMPROVED METHOD FOR CALCULATING VIBRONIC SPECTRA OF COMPLEX-MOLECULES

The previously suggested approximate method for calculating the overla p integrals of vibrational wave functions is considerably improved for the purpose of maximally accurate calculation of excitation-induced m ixing of normal coordinates. A general formula is obtained for all typ es of overlap integrals as a finite power series of the potential surf ace shift parameters; the coefficients are derivatives of the correspo nding generating functions represented as polynomials of the shift vec tor elements of the normal coordinates and the miring matrix. The spec tra of model molecules of decatetraene and tetra-and hexadecaheptaene were calculated using the expressions derived in this work and a semie mpirical parametric method for determination of excitation-induced cha nges in the potential surface of molecules. The calculations confirmed the high efficiency of both the parametric method and the new techniq ue.