Using density functional based tight binding methods in vibrational circular dichroism

Theoretical calculations of vibrational properties are widely used to explain and predict experimental spectra. An example is the interpretation of vibrational circular dichroism (VCD) spectra, for which the absolute configuration of chiral systems can determined by comparing measured and calculated spectra. For large organic molecules that have many assessible conformations, the computational cost of density functional theory (DFT) calculations is considerable and often a limiting factor for practical applications. We have investigated alternatives in which density functional based tight binding (DFTB) is used in parts or all stages of the calculation. DFTB is shown to perform reasonably well at the first stage, yielding frequencies and normal modes in line with DFT data. With standard parametrizations dipole gradients needed in the second stage are, however, poorly reproduced, and improvement using information obtained by DFT calculation is necessary.
As typically only a specific spectral region is of experimental interest, we have thereby developed an efficient method to calculate only a specific part of the spectrum. After the computationally very cheap DFTB estimate of the molecular motions, the computational time is proportional to the number of normal modes needed to describe this frequency range. Results for a medium-sized molecule show a reduction in computational time of up to one order of magnitude with negligible loss in accuracy. We also show that still larger computational savings are possible by using an additional intensity-selection procedure.