AB Initio Calculations Close to the Full CI Level of Accuracy and their Use for the Interpretation of Molecular Spectra

Abstract

A practical method for applying perturbation theory to a completely general multi-reference function is described which is capable of yielding energy results at close to full CI accuracy for all types of electronic states. The advantages of this technique over conventional cluster-type perturbative treatments are pointed out, particularly its self-correcting capability. Various examples of the overall method are presented for different AO basis sets and numbers of electrons, and also for several types of MO or NO transformations. In addition it is discussed how this perturbative CI technique can be generalized to obtain other properties of the associated eigenfunctions. Applications involving current problems of molecular spectroscopy are presented in which the resulting theoretical predictions have been an indispensible aid for the interpretation of the measured data. Furthermore because the method does not rely on a spinless representation of the Hamiltonian it is also possible to extend it in a straight forward manner to include relativistic terms in the treatment, and thus applications are also reported in which effects of this nature have been computed. Finally, molecular calculations which go beyond the Born-Oppenheimer Approximation are also illustrated, with the coupling of vibrational modes being achieved through the explicit computation of non-adiabatic matrix elements over large-scale CI wavefunctions.