Modeling vibrational spectra using the self-consistent charge density-functional tight-binding method. I. Raman spectra

An extension of the self-consistent charge density-functional tight-binding (SCC-DFTB) method is presented that allows for calculating intensities of peaks in vibrational Raman spectra for very large molecules. The extension is based on a simple ansatz: an extra term, which describes interaction of an external electric field with induced atomic charges, is added to the SCC-DFTB energy expression. We apply the modified SCC-DFTB formalism for reproducing vibrational Raman spectra of 17 organic molecules. The calculated spectra are compared with experiment and with spectra obtained from density functional theory (DFT) calculations. We find that the SCC-DFTB method is capable of reproducing most of the features of experimental Raman spectra. Limitations and advantages of this approach are analyzed and suggestions for interpreting calculated SCC-DFTB Raman spectra are given.