Electron CISDT-method

In 1988, Olsen and co-workers46 presented the restricted active space (RAS) Cl, which specifies the Cl space in an a priori manner reminiscent of the second-order Cl (SOCI) and its derivatives. Olsen partitions the orbitals into three subspaces, labeled RAS I, RAS II, and RAS III. Typically, RAS I contains occupied and possibly very important virtual orbitals, RAS II contains the most important virtuals, and RAS III contains the less important virtuals. The Cl space includes all determinants with a minimum of p electrons in RAS I and a maximum of q electrons in RAS III. There is no restriction on RAS II, which is akin to the complete active space. Using this simple procedure, it is possible to formulate any Cl space truncated according to excitation level (e.g., CISD, CISDT, etc.) as well as excitation class selected MR-CI spaces, such as SOCI and CISD[TQ], The RAS Cl method is discussed more fully in section 4.8. 

The trial wavefunction can include the exchange of 1, 2 or 3 electrons from the valence band into unoccupied orbitals these are known as Cl singles (CIS), Cl doubles (CID) and Cl triples (CIT), respectively. CIS, CISD, and CISDT are methods configurational... 

Develop some procedure for selection of the most important con%urations. In its most general form this method is called GenCI. Frequently used are CIS, CID, CISD, CISDT etc. methods where 1-electron, 2-electron, 1+2-electron, 1+2+3-electron excitations of electrons from occupied HF states to virtual ones are taken into account. These methods can be called restricted Cl (RCI) methods. In some computer codes for molecular calculations automatic selection of the most important configurations is performed [119]. 

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