Improved method for calculating vibronic spectra of complex molecules

The previously suggested approximate method for calculating the overlap integrals of vibrational wave functions is considerably improved for the purpose of maximally accurate calculation of excitation-induced mixing of normal coordinates. A general formula is obtained for all types of overlap integrals as a finite power series of the potential surface shift parameters; the coefficients are derivatives of the corresponding generating Junctions represented as polynomials of the shift vector elements of the normal coordinates and the mixing matrix. The spectra of model molecules of decatetraene and tetra- and hexadecaheptaene were calculated using the expressions derived in this work and a semiempirical parametric method for determination of excitation-induced changes in the potential surface of molecules. The calculations confirmed the high efficiency of both the parametric method and the new technique.