Multiconfigurational quantum chemistry for ground and excited states

One frequently used quantum chemical approach for studies of spectroscopy and photochemistry is the Complete Active Space (CAS) SCF method in combination with multiconfigurational second order perturbation theory (CASPT2). In this chapter we shall describe these two approaches. The basic idea behind them is the request that the wave function should give a proper description of the electronic structure already at the lowest level of theory. This should be possible for all possible arrangements of the electrons: in chemical bonds, in excited states, in dissociated states, at transition states for chemical reactions, etc. It should also be possible for all atoms of the periodic systems. The CASSCF wave function fulfills, in principle, this requirement because it is full CI, albeit in a limited space of active orbitals. CASSCF can therefore be regarded as an extension of the Hartree-Fock (HF) method to any arrangement of the electrons. The addition of dynamic electron correlation is as crucial here as it is in the HF method. The suggested solution is to compute this energy using second order perturbation theory (CASPT2) because it is relatively simple and allows applications to a wide variety of systems and many electrons. The review will focus on the methods themselves. Applications will be described in other chapters of the book