State-averaged MCSCF theory

So far in our discussion of Cl and MCSCF theory, we have found it convenient to employ a redundant parametrization of the configuration space - see Section 11.4.1 for the (3 wave function and Section 12.2.1 for the MCSCF wave function. This parametrization has the advantage of simplicity, which for many purposes such as the optimization of the wave function offsets the disadvantages arising from the presence of the redundancy. Nevertheless, in some situations, it is best to remove this redundancy and to use a parametrization where the electronic states are described by a set of independent, nonredundant variables. One such situation arises in the state-averaged MCSCF scheme discussed in Section 12.7, where several MCSCF states are generated simultaneously in the same orbital basis. 

Multiconfigurational quasi-degenerate perturbation theory (MC-QDPT) [5,6] We have also proposed a multistate multireference perturbation theory, the QDPT with MCSCF reference functions (MC-QDPT). In this PT, state-averaged CASSCF is first performed to set reference functions, and then an elfective Hamiltonian is constructed, which is finally diagonalized to obtain the energies of interest. 

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