The importance of electron correlations on vibrational anharmonicities and potential energy surfaces

An assessment and importance of different-order of electron correlation effects on anharmonic vibrational potential energy surfaces (PESs) is reported. Systematic construction of quadratic, cubic and quartic force-fields using CCSD(T), MP2 and HF methods are performed for anharmonic vibrational spectroscopic calculations. The accuracies and convergences are assessed for VPT2 and VCI methods. The level of electron correlation is found important for the accuracies of harmonic force-constants while its effects on anharmonicities are less significant. Following this, suitable hybrid PESs are suggested for better accuracy/cost ratio. For the hybrid PESs, a more accurate high-level and less accurate low-level electron- correlation methods are employed to compute the harmonic and anharmonic part of the potential, respectively. The estimated deviations for hybrid PESs are found a few wavenumbers with the highly correlated methods as well as experiment. That in turn shows the validity and applicability of such hybrid PESs for large molecular systems.