Note

This website is presently under development. The primary website for MOPAC is still http://openmopac.net until the majority of its contents have been migrated here.

C.I.

Defines an active space for multi-electron configuration interaction (MECI) calculations from orbitals near the chemical potential. By default, this keyword defines a complete active space, but the active space can be further restricted to specific classes of excitations with the keywords CIS, CISD, and CISDT.

SYNTAX: C.I.=n

Activate the \(\lceil n/2 \rceil\) orbitals at or below the chemical potential and the \(\lfloor n/2 \rfloor\) orbitals above the chemical potential.

SYNTAX: C.I.=(n,m)

Activate the \(n\) orbitals closest to the chemical potential containing \(m\) electron pairs. This accounting ignores the unpaired electron in systems with an odd number of electrons.

MOPAC constructs and diagonalizes the active-space Hamiltonian as a dense matrix, which becomes prohibitive in cost around C.I.=8 if the active space is not restricted in size with other keywords.

Number of configurations in MECI active spaces

Keyword

Even number of electrons

Odd number of electrons

1 variable

2 variables

# of \(\alpha\)

# of \(\beta\)

# of configs

# of \(\alpha\)

# of \(\beta\)

# of configs

C.I.=1

C.I.=(1,1)

1

1

1

1

0

1

C.I.=2

C.I.=(2,1)

1

1

4

1

0

2

C.I.=3

C.I.=(3,2)

2

2

9

2

1

9

C.I.=4

C.I.=(4,2)

2

2

36

2

1

24

C.I.=5

C.I.=(5,3)

3

3

100

3

2

100

C.I.=6

C.I.=(6,3)

3

3

400

3

2

300

C.I.=7

C.I.=(7,4)

4

4

1225

4

3

1225

C.I.=8

C.I.=(8,4)

4

4

4900

4

3

3920

Note

INDO calculations use this keyword to specify the active orbitals for single-electron excitations, either from a Hatree-Fock reference state (CIS or CISD) or from a complete active space (MRCI). By default, all orbitals are active for this purpose.