CASSCF calculations

The complete active space self-consistent field (CASSCF) method requires some chemical insight of the problem under investigation. The setup of the calculation requires a careful design of the active space. The active space consists of the electrons and orbitals that are necessary for a reliable description of the electronic structure of a molecule or to describe the reaction that is being studied.

You can almost never start a CASSCF-type calculation using orbitals automatically created by the ORCA guess as is done for standard SCF calculations. Typically, you need to spend some time thinking about your active space and what orbitals should be in the active space. Previously calculated DFT orbitals (often QROs) or MP2 natural orbitals are usually used as CASSCF guess orbitals or orbitals from a previous CASSCF calculation. See Tutorial: Setting up the orbitals for a CAS calculation .

Minimal input

The basic input requires the number of n active electrons and m active orbitals, denoted by CASSCF(n,m) or shortly CAS(n,m). The example below demonstrates the input for the case of CAS(4,3):

! RHF def2-SVP

%casscf
nel 4
norb 3
mult 1
end

The nel keyword in the %casscf block is used to specify the number of active electrons, while norb specifies the number of active orbitals. Furthermore, the multiplicity is given by the mult keyword. This renders the minimum input for a ground state calculation. Additional roots have to be included for excited state calculations. Then the number of roots (which corresponds to the number of states) is assigned by the nroots keyword (see State-averaging section below).

Sometimes a CAS-CI calculation is wanted where the orbitals are not optimized. This is performed by adding the NoIter keyword:

! RHF def2-SVP noiter

%casscf
nel 4
norb 3
mult 1
end

State-average CASSCF wavefunction

! RHF def2-SVP

%casscf
nel 4
norb 3
mult 1,3
nroots 2,2
end

In this example two multiplicity blocks are specified and two states are selected for each multiplicity. As a consequence the CASSCF wavefunction calculate 4 states and the orbitals will be averaged for two singlet states and two triplet states. When selecting multiple roots like this, the weight of each root (i.e. the contribution to the state-averaging) should be specified. The default is to make an equal weighting for each root.

Weights:
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NEVPT2

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CASSCF Properties

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