MRCI calculations are invoked using the

`mrci`

or

`mrcic`

directive. `mrcic` is a new program that is much more efficient than
`mrci` for cases with many inactive (closed-shell) orbitals in the reference function.
Currently, it is still restricted to single-state calculations. Note that `mrci`
and `mrcic` give sligfhtly different results if inactive orbitals are present,
since `mrcic` uses a more strongly contracted wave function ansatz [see
K. R. Shamasundar, G. Knizia, and H.-J. Werner, J. Chem. Phys. **135**, 054101 (2011)].
See also below for `rs2c`, which uses the same ansatz.

By default, the same occupied and closed-shell spaces as in the preceding
MCSCF (CASSCF) calculation are used, and the inner-shell *core* orbitals are
not correlated (i.e., the orbitals of carbon or oxygen, or the , , and
orbitals in chlorine). The number of uncorrelated core orbitals can be modified
using the `core` directive (see section 6.1). It is not necessary
that the reference wavefunction is exactly the same as in the MCSCF, and the `occ`,
`closed`, `restrict`, and `select` directives can be used exactly in the
same way as explained for MCSCF and CASSCF.

By default, the orbitals are taken from the most recent orbital optimization calculation
(HF or MCSCF/CASSCF). Other orbitals can be specified using the `orbital` directive.
Please refer to the reference manual for further details.

The following is an example of a CASSCF/MRCI calculations for .

***,O2 print,orbitals,civector !print orbitals and ci-coefficients geometry={ !geometry specification, using z-matrix o1 o2,o1,r } r=2.2 bohr !bond distance basis=vtz !triple-zeta basis set {hf !invoke RHF program wf,16,4,2 !define wavefunction: 16 electrons, symmetry 4, !triplet occ,3,1,1,,2,1,1 !number of occupied orbitals in each symmetry open,1.6,1.7} !define open shell orbitals casscf !casscf using full valence active space mrci !mrci using full valence casscf reference function {mrci !mrci with only 2p orbitals active, 2s closed !in reference closed,2,,,,2} !inactive orbitals in the reference function.

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molpro@molpro.net 2018-06-19