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7.4 MCSCF with selected configurations

MCSCF with arbitrary selected configuration spaces can also be performed. The only restriction is that always all CSFs with different spin couplings arising from the same orbital occupancy are included. There are two ways to select configurations. Either, they are selected from a previous calculation with a threshold, or they are defined explicitly in the input. Here we only describe the latter case; for more details please refer to the reference manual.

The configuration input starts with the select directive, and subsequently each orbital configuration is given by one line starting with the keyword con, followed by the occupation numbers of the active orbitals. The following example shows an input for formaldehyde.

geometry={                !define the nuclear coordinates
O  , C , rco
H1 , C , rch , O , hco
H2 , C , rch , O , hco , H1 , 180

rco=1.182 Ang
rch=1.102 Ang
hco=122.1789 Degree

basis=vdz              !Select basis set
hf                     !Perform HF calculation
{mcscf                 !invoke MCSCF program
closed,4,,1            !4a1 and 1b2 inactive orbitals
occ,6,2,3              !6a1, 2b1, 3b2 occupied orbitals
wf,16,1,0              !16 electrons, Symmetry 1 (A1), singlet
select                 !start configuration selection
con, 3.1,-3.1,1.2,-1.2,1.3,-1.3     !occupied orbitals in 1st configuration
con, 3.1,-3.1,2.2,-2.2,1.3,-1.3
con, 3.1,-3.1,1.2,-1.2,2.3,-2.3
con, 3.1,4.1,1.2,2.2,1.3,-1.3
con, 3.1,-3.1,1.2,2.2,1.3,2.3
con, 4.1,-4.1,1.2,-1.2,1.3,-1.3}

This calculation includes all CSFs of the CASSCF with coefficients larger than 0.04. 2017-12-12