CASSCF: selecting active space
 
To select the active space in CASSCF calculations or to chose the active space options in a RASSCF calculation it is often said that we should be guided by our chemical intuition. In this topic I would like to collect some ideas that can be used to chose the active space. I will try to list them from  
 

• The active space should contain all MO's that have the same character as the HOMO or the orbital that you think is important. For example in a pi system you should look at all orbitals with pi character.
  
• Include MO's that are close to eath other in energy
  
• Look at the occupation numbers of the natural orbitals. If they differ from 2 (fully occupied) or 0 (unoccupied) they signify it is an important MO to include in the active space. A small amount of electron occupation is rougly analogous to the excited configuration weight.  
  
• Inspect the MO's of the starting orbitals with that of the RASSCF solution. They should be similar in nature.
  
• Look at twisted molecule geometries to highlight configurations that may become important in deformations.
  
• Do a CASPT2 calculation. If there are states that are not included in the active space but that are close in energy to the reference state the CASPT2 may diverge and complain of intruder states. You should try to include the intruder state into the active space.