Chemical Intuition and Bjorns Rules

There are many factors governing the choice of active space. An important consideration is the nature of the chemical problem that is to be addressed. For example, if a chemical reaction is to be studied, then all orbitals involved in bond breaking/formation must be considered. If, instead, excited state properties are of interest, for example in the simulation of absorption processes, then orbitals whose occupations differ significantly between the ground and excited state must be included. This can be particularly important in coordination complexes, where excitation processes may involve a change in the oxidation state of the metal ion. However, some general points with regard to the choice of active space orbitals can be made  

Bjorn Roos attempted to develop mles to aid in the construction of active spaces, and some of these rules are briefly summarised here  

The 5f, 6d, and 7s orbitals of the actinides should be included (although this is often impractical). Only 5f orbitals need be included for highly charged complexes. 

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