Computations of the Raman Optical Activity via the sum-over-states expansions

A computationally convenient and reasonably accurate scheme of computation of the Raman Optical Activity (ROA) is presented and tested on model exampl es. Electromagnetic tensors were obtained using the sum-over-states (SOS) m ethodology, while their nuclear derivatives were estimated through numerica l differentiation. An origin dependence of the results was overcome by a di stributed origin gauge transformation. Becke-3LYP functional and correspond ing Kohn-Sham orbitals are used for the excited states. The method was comp ared to a benchmark coupled-perturbed (CP) calculation on formamide and a s tandard ROA spectral simulation and experiment for alpha -pinene. Spectra o f four standard peptide conformations (alpha -helix, 3(10)-helix, coil, and beta -sheet) were simulated with smaller fragments and compared to previou s experimental observations. (C) 2001 John Wiley & Sons, Inc.