Fast degenerate correlation-corrected vibrational self-consistent field calculations of the vibrational spectrum of 4-mercaptopyridine.

We introduce a fast degeneracy-corrected vibrational second-order Møller-Plesset (fast-DCVMP2) method to compute anharmonic vibrational spectra of large molecules where the computational cost of the full potential energy surface is high. We examine the suitability of the non-self-consistent Harris functional as a robust prescreening technique to replace the semiempirical PM3 model used in our previous studies. We analyze the mode-mode coupling strength statistically and present a scheme that provides a more flexible determination of the threshold used to identify strong couplings. Our methodology is validated on the methanol molecule and applied to mercaptomethane and pyridine, where we compare our results with experimental frequencies. We show that a standard perturbative correction of the vibrational self-consistent field energy can lead to unreliable results even for systems where degeneracies are not expected to play an important role. Our fast technique leads to results close to those obtained with standard DCVMP2 calculations, but with substantial time savings. Using this new technique, we compute the vibrational frequencies of 4-mercaptopyridine, an interesting compound for molecular-electronic applications, and compare our results with experimental values.