THE EXCITATION SCHEME - A NEW METHOD FOR CALCULATION OF VIBRATIONAL CIRCULAR-DICHROISM SPECTRA

An alternate procedure fur calculating vibrational circular dichroism is proposed, The method eliminates the need to solve the magnetically perturbed SCF equations which leads to an overall decrease in computer time. Instead, an expansion over electronic excited states is partial ly used to estimate the molecular response to the magnetic field and n uclear displacements. A rigid orbital approximation is used for the el ectronic states. The rotational strengths obtained in this manner are compared to original experimental data for camphor and cr-pinene, and previous calculations on propylene oxide and oxirane. In all cases goo d agreement of simulated spectral intensities with experiment is obser ved. Although extensive approximations had to be adopted in the curren t implementation, the excitation scheme yields results superior to tho se obtained by the classical MFP or VCT formulations of VCD for campho r and alpha-pinene: whereas MFP/GIAO theory performs better in the cas e of oxirane. (C) 1998 American Institute of Physics. [S0021-9606(98)0 1321-X]