Polarization directions of electronic transitions in large conjugated molecules of low symmetry

Advances in experimental techniques for determination of transition moment directions in large molecules call for evaluation of the reliability of angles calculated using the semiempirical SCF-CI PPP method. Two disturbing features are considered: first, the large difference between oscillator strengths calculated by the dipole length and dipole velocity methods; second, the considerable effect of more highly excited configurations on the calculated transition moments and gradients. The two problems can be handled simultaneously if matrix elements of the linear momentum operator are obtained from the resonance integrals, since then the two operators give identical answers using wave functions derived from complete configuration interaction (CI). Numerical investigation on model systems is made of the extent of CI needed for such convergence. For strong transitions, results for polarization directions already reach their limiting values when very limited CI is performed.