Black-Box Simulations of Anharmonic Vibrational Chiroptical Spectra: Problems with Property Third Derivatives and the Solvent.

Chiroptical methods, including vibrational circular dichroism (VCD) and Raman optical activity (ROA), reveal details about molecular structure. For three model molecules, α-pinene, camphor, and fenchone, we show that increased sensitivity of modern spectrometers makes it possible to record even fine spectral features, such as overtone and combination bands. However, understanding, interpretation, and simulation of them require relatively expensive computations, going beyond the harmonic approximation. For this purpose, vibrational perturbation theory at the second order (VPT2) has proven to provide an excellent price-performance balance. As it becomes more common, inconsistencies in electronic structure calculations, hidden by error compensation at the harmonic level, emerge. In particular, while trying to interpret the spectra, we found that the commonly used polarizable continuum models (PCM) of solvent may introduce erroneous perturbations to the higher derivatives of dipole moments and polarizabilities needed to simulate spectral intensities. We therefore analyze the experimental spectra on the basis of the simulations and explore parameters allowing for a "black-box" VPT2 application. In particular, explicit cavities used for the hydrogen atoms resulted in excessively large third derivatives of molecular polarizabilities and sometimes led to incorrect signs of ROA and VCD bands, even for fundamental transitions. This could be partially rectified by a combination of different approximation levels used for the calculation of different properties, or by using PCM cavities not explicitly adapted for hydrogen atoms. Under these conditions, VPT2 combined with a proper treatment of resonances appears as an excellent tool to simulate and understand the spectra, including the assignment of weak anharmonic bands.