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# CASSCF

CASSCF calculations can be performed with CFOUR via

CALC=CASSCF

Full CI calculations, with and without Frozen Core, can be also performed using the CASSCF module via

CALC=FULLCI

At the moment, only the CASSCF energy can be computed. The CASSCF module requires additional input, in particular, the user needs to specify the active space and how many alpha and beta electrons are to be correlated. The CASSCF input is given via an additional %casscf section which contains the following information

- number of alpha/beta electrons and number of orbitals
- definition of the active space.

The number of active electrons and orbitals is specified by three integers separated by a space and given in the order
"number of alpha electrons" "number of beta electrons" "number of orbitals".
The definition of the active space can be provided in two different ways:

• after running a HF calculation, the active orbitals can be given by providing a list of orbital numbers, as printed after the SCF has converged (in energy order). If an unrestricted calculation is performed, the alpha orbitals are to be used to specify the active space.
• by specifying the keyword CAS_INPUT=OCCUPATION, the active space can be defined by providing the number of internal orbitals per irrep and, on a different line, the number of active orbitals per irrep.

As an example, we consider the SCF results for the OH- anion.

  ORBITAL EIGENVALUES (ALPHA)  (1H = 27.2113838 eV)

MO #        E(hartree)               E(eV)           FULLSYM    COMPSYM
----   --------------------   --------------------   -------   ---------
1     1         -19.9311866358        -542.3551695258     SG+        A1 (1)
2     2          -0.7546487737         -20.5350374298     SG+        A1 (1)
3     3          -0.0982048749          -2.6722905436     SG+        A1 (1)
4     8          -0.0026260430          -0.0714582650      PI        B1 (2)
5    10          -0.0026260400          -0.0714581834      PI        B2 (3)
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
6     4           0.6343389513          17.2612406758     SG+        A1 (1)
7     5           1.5405378884          41.9201677701     SG+        A1 (1)
8     9           2.2417638388          61.0014962509      PI        B1 (2)
9    11           2.2417638422          61.0014963424      PI        B2 (3)
10     6           2.4059031203          65.4679532399     SG+        A1 (1)
11     7           3.2891549689          89.5024583010     SG+        A1 (1)


The CASSCF input for a CAS(8,8) calculation (full valence CAS) on the anion can be given as either

%casscf
4 4 8
2 3 4 5 6 7 8 9

or, by adding the keyword CAS_INPUT=OCCUPATION, by

%casscf
4 4 8
1 0 0 0
4 2 2 0

Notice that the SCF orbitals are used as a guess. A calculation on the OH radical can be performed by using the same orbitals and the following input

%casscf
4 3 8
2 3 4 5 6 7 8 9
or, alternatively (with CAS_INPUT=OCCUPATION), by

%casscf
4 3 8
1 0 0 0
4 2 2 0
In other words, there is no need (although it is possible) to perform an unrestricted calculation on the OH, as the SCF orbitals are merely used as a guess. Also, the casscf input overrides the CHARGE and MULT input.

The convergence threshold for the CASSCF optimization can be specified using the CC_CONV keyword.