Multireference Perturbation Theories for the accurate calculation of energy and molecular properties

This Ph.D. thesis deals with the development and the applications of N-Electron Valence State Perturbation Theory (NEVPT), a novel form of Multireference Perturbation Theory (MRPT) put forward in collaboration between the theoretical chemistry groups of the uni- versities of Ferrara and Toulouse. A review of the NEVPT approach is presented, starting from the original second order state–specific formulation, going through the quasidegenerate multi–state extension and arriving at the implementations of the third order in the energy and of the internally contracted configuration interaction, accomplished during the Ph.D. The chief properties of NEVPT are analyzed and the test case of the C2 molecule is discussed. An important field of applications of MRPTs is the calculation of the electronically excited states of molecules, where the strong differential correlation effects and the possible multiref- erence nature of the wavefunctions can be, in principle, successfully handled by a “variational plus perturbation” scheme. Concerning the applications, the thesis is divided in two parts. Part I concerns the calculation of electronically excited states. Different issues are addressed: on the one hand the treatment of small aromatic molecules, Pyrrole, Furan and Thiophene, whose description is complicated by the possible interaction with low-lying Rydberg states and by the ionic nature of some valence states, extremely sensitive to the so-called dynamical − polarization; on the other hand the case of a large-sized aromatic molecule, Free-Base Por- phin, for which the crucial problem is the choice of a balanced variational space to accurately describe the wavefunctions of the ground and of the excited states. The second part is devoted to the description, by means of MRPT, of the Electron Transfer (ET) process in Mixed-Valence systems. The investigation is carried out on a model spiro − − compound, for which the ET reaction is simulated using a simplified one-mode two-state model. The inadequacy of a standard second order MRPT approach is shown and the application of an alternative and effective computational strategy is discussed.