The accurate calculation of molecular properties by ab initio methods

Abstract

Advances are reported on three fronts for the ab initio prediction of molecular properties: (i) our program for the calculation of electronic properties using second-order perturbation theories for the inclusion of electron correlation (Moller–Plesset-2, or MP2) is now fully operational. We may optimise geometries, calculate harmonic frequencies and infrared intensities using large basis sets; (ii) we may calculate by analytical methods anharmonic effects using the self-consistent field (SCF) method. Analytic third derivatives are fully operational, and fourth derivatives will soon be available. This leads the way to the ab initio determination of quartic potential-energy surfaces; (iii) once a potential surface is available, we may also calculate the rovibrational energy levels using the variational method. The advance reported here is the extension to tetra-atomic molecules, using internal coordinates throughout. As an example of all three methods, we report detailed calculations on acetylene, C₂H₂, together with a comparison to experimental information. The first two approaches are readily applicable and ideally suited for the study of weakly bound complexes. We also report a study of C₂H₂…HF. As a final example, we report some calculations for Fermi resonance constants.