General perturbative approach for spectroscopy, thermodynamics and kinetics: methodological background and benchmark studies

A general second-order perturbative approach based on resonance- and threshold-free computations of vibrational properties is introduced and validated. It starts from the evaluation of accurate anharmonic zero-point vibrational energies for semirigid molecular systems, in a way that avoids any singularity. Next, the degeneracy corrected second-order perturbation theory (DCPT2) is extended to a hybrid version (HDCPT2), allowing for reliable computations even in cases where the original formulation faces against severe problems, including also an automatic treatment of internal rotations through the hindered-rotor model. These approaches, in conjunction with the so-called simple perturbation theory (SPT) reformulated to treat consistently both energy minima and transition states, allow one to evaluate degeneracy-corrected partition functions further used to obtain vibrational contributions to properties like enthalpy, entropy, or specific heat. The spectroscopic accuracy:of the HDCPT2 model has been also validated by computing anharmonic vibrational frequencies for a number of small-to-medium size, closed- and open-shell, molecular systems, within an accuracy close to that of well established but threshold-dependent perturbative-variational models. The reliability of the B3LYP/aug-N07D model for anharmonic computations is also highlighted, with possible improvements provided by the B2PLYP/aug-cc-pVTZ models or by hybrid schemes. On a general grounds, the overall approach proposed in the present work is able to provide the proper accuracy to support experimental investigations even for large molecular systems of biotechnological interest in a fully automated manner, without any ad hoc scaling procedure. This means a fully ab initio evaluation of thermodynamic and spectroscopic properties with an overall accuracy of about, or better than, 1 kJ mol(-1), 1 J mol(-1) K-1 and 10 cm(-1) for enthalpies, entropies, and vibrational frequencies, respectively.