Estudos ab initio de alta precisão aplicados à pequenas moléculas

Abstract

The objective of this work was to calculate molecular properties of small systems using to that end only first principles (ab initio) in its state-of-the-art. Two systems were studied: Si3, which in decades has receiving attention from the scientific community after it wasdetected in stars spectra and later in the application on the semiconductors area; and the CNO system, whose components plays a major role in the atmospheric chemistry. The silicon trimer has a complex electronic states configuration, and in this work several were characterized. A special attention was given to the two lowest states (X1A1) and (3B2) and their spin-orbit coupling was calculated. There was some doubts in the literature about which was the true ground state and in this work it become clear that the it is the (X1A1). Data regarding geometries, vibrational frequencies and relative energies are also updated to a more recent level as the Muti-Reference Configuration Interaction, including the correction to the Complete Basis Set Limit.A global potential energy surface was constructed to the CNO ground state (X2A), whose energies were obtained from Explicitly-Correlated Multi-Reference Configuration Interaction calculation and fitted to the functional form of the Double Many-Body Expansion method. Several important saddle points and their frequencies were characterized. Thesurface is developed to calculations of reactive dynamics and kinetics.