When C.I.=n is specified, the n M.O.s which 'bracket' the occupiedvirtual energy levels will be used in a configuration interaction calculation. Thus, C.I.=2 will include both the HOMO and the LUMO, while C.I.=1 (implied for oddelectron systems) will only include the HOMO (This will do nothing for a closedshell system, and leads to Dewar's halfelectron correction [136] for oddelectron systems). Users should be aware of the rapid increase in the size of the C.I. with increasing numbers of M.O.s being used. Normally, configuration interaction (see the MECI) is invoked if the keywords which imply a C.I. calculation are used (BIRADICAL or EXCITED). Note that UHF should not be present.
Table: Number of Configurations used in MECI

No. of Electrons 
Configs 
No. of Electrons 
Configs 


Alpha 
Beta 

Alpha 
Beta 

C.I.=1 
1 
1 
1 
1 
0 
1 
C.I.=2 
1 
1 
4 
1 
0 
2 
C.I.=3 
2 
2 
9 
2 
1 
9 
C.I.=4 
2 
2 
36 
2 
1 
24 
C.I.=5 
3 
3 
100 
3 
2 
100 
C.I.=6 
3 
3 
400 
3 
2 
300 
C.I.=7 
4 
4 
1225 
4 
3 
1225 
C.I.=8^{*} 
4 
4 
4900 
4 
3 
3920 
*: Do not use unless other keywords are also used. See MICROS, and the discussion in Choice of State to be Optimized .
If a change of spin is defined, then larger numbers of M.O.s can be used up to a maximum of 22. If more than 22 M.O.s are needed, then meci_C.F90 would need to be changed. In meci_C.F90, increase the value of NMECI as necessary. Normally, a full C.I. is carried out, in which case the spinstates are exact eigenstates of the spin operators. For systems with more than the normal number of configurations (Table), the configurations of lowest energy will be used. See also MICROS and the keywords defining spinstates.
Note that for any system, use of C.I.=5 or higher normally implies the diagonalization of quite large matrices. As a geometry optimization using a C.I. requires the derivatives to be calculated using derivatives of the C.I. matrix, geometry optimization with large C.I.s will require more time than smaller C.I.s.
If microstates are supplied, using MICROS, then the states used are precisely controlled. This requires more work, because each microstate has to be explicitly defined, but it is very flexible and much more certain than when other keywords such as CISDT are used.
Associated keywords: OPEN(n_{1},n_{2}), C.I.=(n_{1},n_{2}), CIS, CISD, PECI, CISDT, MECI, ROOT, MICROS, SINGLET, DOUBLET, etc.