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8.1 Multireference configuration interaction (MRCI)

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 $1s$ orbitals of carbon or oxygen, or the $1s$, $2s$, and $2p$ 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 $\rm O_2$.

***,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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