Computation of full infrared (IR) and Raman spectra (including absolute intensities and transition energies) for medium- and large-sized molecular systems beyond the harmonic approximation is one of the most interesting challenges of contemporary computational chemistry. Contrary to common beliefs, low-order perturbation theory is able to deliver results of high accuracy (actually often better than those issuing from current direct dynamics approaches) provided that anharmonic resonances are properly managed. This perspective sketches the recent developments in our research group toward the development of a robust and user-friendly virtual spectrometer rooted in second-order vibrational perturbation theory (VPT2) and usable also by non-specialists essentially as a black-box procedure. Several examples are explicitly worked out in order to illustrate the features of our computational tool together with the most important ongoing developments.

A general and effective time-independent approach to compute vibrationally resolved electronic spectra from first principles has been integrated into the Gaussian computational chemistry package. This computational tool offers a simple and easy-to-use way to compute theoretical spectra starting from geometry optimization and frequency calculations for each electronic state. It is shown that in such a way it is straightforward to combine calculation of Franck-Condon integrals with any electronic computational model. The given examples illustrate the calculation of absorption and emission spectra, all in the UV-vis region, of various systems from small molecules to large ones, in gas as well as in condensed phases. The computational models applied range from fully quantum mechanical descriptions to discrete/continuum quantum mechanical/molecular mechanical/polarizable continuum models.

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.

An effective time-independent approach to compute vibrationally resolved optical spectra from first principles is generalized toward the computation of one-photon electronic spectra induced by either electric or magnetic transition dipoles or by their mutual interaction. These encompass absorption, emission, and circular dichroism spectra. Additionally, the proposed computational scheme is extended to cover a broad range of approximations to evaluate vibronic transitions within both vertical and adiabatic frameworks and to be able to take into account the effects of the temperature. The presented computational tool is integrated into a general purpose computational chemistry package and offers a simple and an easy-touse way to evaluate one-photon electronic spectra, starting from electronic structure calculations chosen according to the system under study, from fully quantum mechanical descriptions to discrete/continuum quantum mechanical/MM/polarizable continuum models.