Vibrational mode tailoring approach: an efficient route to compute anharmonic molecular vibrations of large molecules

In this study, an efficient route to compute the multidimensional potential energy surfaces (PESs) for the description of quantum anharmonic molecular vibrations is presented. For large molecular systems, where the number of inter mode coupling terms are substantial, a tailor-made construction of truncated PESs is suggested which suitably avoids computation of full PESs by quantitative assessment of the atomic displacements during the normal mode of vibrations. It is shown that, typically, when two normal modes of vibrations are sharing the same atoms, the mode-mode coupling strength is generally large and can be included for truncated PESs. Since the calculations of these terms are the main computational bottleneck, the guided construction of tailor-made PESs can remarkably speed up the calculation by many folds against the expense of little accuracy for peak positions and intensities. This protocol is applicable to any anharmonic vibrational algorithm and more appropriate for large molecules where exploration of chemically important small fragments is more significant compared to that of the entire molecule. A systematic study for n-butanol and a dipeptide in the framework of the VSCF-PT2 method shows that for a group of n target modes, inclusion of a set of 5n-6n other associated important modes is sufficient to offer good accuracy with an error of ∼5-20 cm^-1 against the full PES and computationally faster by ∼10-20 times. Finally, a prescription is given on the choice of such tailor-made PESs to compute anharmonic vibrational spectra accurately and efficiently without calculating the full PES.