MULTICONFIGURATIONAL 2ND-ORDER PERTURBATIVE METHODS - OVERVIEW AND COMPARISON OF BASIC PROPERTIES

The multiconfigurational second-order perturbative treatments of molec ular electronic calculations can be classified into four groups: i) qu asi-degenerate perturbation theory (QDPT) in the basis of determinants , ii) non-degenerate perturbation theory applied to eigenvectors resul ting from a truncated CI, ii) QDPT in a model space of non-interacting multiconfigurational functions, iv) intermediate Hamiltonians theory, and examined according to three criteria: i) risk of numerical instab ility due to intruder states, ii) ability to treat the effect of the o uter-space on the model space component of the wavefunction, especiall y important for the treatment of weakly avoided crossings, iii) separa bility for (A...B) problems. None of the existing methods satisfies t hese three criteria, as shown both by model analysis and real ab initi o calculations on LiF and CuF.