Item

Abstract

The main goal of this work was the development of theoretical methods for calculating the rotational-vibrational spectra and properties of small molecules in the ground electronic state. The functionality of the computer program TROVE, devised for variational calculations of the rotational-vibrational states, was extended in three major areas: (i) calculation of accurate potential energy surfaces (PES) by refinement against spectroscopic data, (ii) calculation of high-resolution rotational-vibrational spectra, (iii) calculation of rotational-vibrational and temperature corrections to electromagnetic properties, as well as high-temperature partition functions and related thermodynamic properties.
PES. In our work we have developed, based on the variational TROVE method, an efficient method for refining an ab initio PES to spectroscopic data, which allows us to treat up to penta-atomic molecules. It combines the high flexibility of the quantum chemical methods in predicting global PESs with the high accuracy of the spectroscopic methods. Using this approach we have obtained PESs for NH₃ and H₂CO of unprecedented accuracy unreachable for state-of-the-art quantum chemical methods so far.
SPECTRA. We have developed and implemented in TROVE a general method for calculating the transition line strengths and intensities for arbitrary molecules. Due to algorithmic improvements the computations have become feasible for tetra- and penta-atomic molecules and for spectra comprising millions of lines. We have applied this method for calculating the spectra of HSOH, SbH₃, and NH₃. For NH₃ we have produced an astrophysical line list consisting of 3.25 million transitions between states with energies up to 12 000 cm^-1.
PROPERTIES. We have addressed the problem of calculating the rovibrational contributions to molecular properties and developed two such approaches. The first one treats the rovibrational and temperature contributions variationally, by averaging the corresponding electronic property over the rovibrational states. Our study of the rovibrational contributions to the indirect nuclear spin-spin coupling constants of ammonia has shown that the effects of large amplitude motion and Coriolis coupling are quite substantial and cannot be recovered even with a high-order perturbation treatment. The second approach directly deals with the spectroscopic observables, namely rovibrational energy levels, their derivatives, and the spectra of a molecule in an electromagnetic field.