Economical description of electron correlation

In this section two approaches to the selection of CSF expansion spaces for MCSCF wavefunctions have been described. Most modern MCSCF methods are best suited to the a priori selection of CSFs based on orbital occupation and spin-coupling restrictions. These a priori selection approaches should be accompanied with empirical evidence (usually more extensive MCSCF or Cl calculations at selected geometries) that a balanced description of the molecular electron correlation is achieved. A reasonable approach to use for molecular systems is to begin with an RCI expansion of all the valence electrons. The less important electrons may then optionally be described with the more restrictive, and more economical, PPMC expansion. The description of the more important electrons in the RCI expansion may be generalized, if necessary, to other direct product type expansions, thereby allowing a more thorough treatment of the correlation of these electrons. 

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