Multiconfiguration Self-Consistent Field and Multireference Configuration Interaction Methods and Applications

We have discussed the relevant aspects of the MCSCF and the MRCI methods which are generally applicable procedures to compute approximations to the electronic Schr€odinger equation. These multireference methods share the important asset that they are not inherently tied to some restricted reference state, and thus they are applicable to arbitrary electronic states and molecular geometries. The variational nature of these methods greatly simplifies the formulation and the implementation of analytical gradients and nonadiabatic coupling vectors. While MCSCF is primarily used to optimize the one-electron basis functions and to describe static electron correlation effects due to nearly degenerate electronic states, the subsequent MRCI method aims at quantitative treatment of dynamical electron correlation. The most prominent restrictions of traditional CI procedures are the lack of size-consistency and the exponential growth of the N-particle expansion space. The latter initiated the deve