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6.2 Open-shell correlation methods

For open shell the following single-reference methods are implemented in MOLPRO:

ump2    !second-order Moeller-Plesset perturbation theory with UHF reference
rmp2    !second-order Moeller-Plesset perturbation theory with RHF reference.
ci      !singles and doubles configuration interaction (using the MRCI program)
rcisd   !spin-restricted singles and doubles configuration interaction
ucisd   !spin-unrestricted singles and doubles configuration interaction
rccsd   !partially spin adapted coupled cluster with singles and doubles
rccsd(t)!partially spin adapted coupled cluster with perturbative
        !treatment of triples
uccsd   !spin-unrestricted coupled cluster with singles and doubles
uccsd(t)!spin-unrestricted coupled cluster with perturbative
        !treatment of triples
Again, there are also explicitly correlated and local variants of many of these methods, see sections 18 and 17, respectively.

Note that all methods except ump2 use spin-restricted Hartree-Fock (RHF) reference functions. This is also the case for the ucisd and uccsd methods, in which the correlated wavefunction may be slightly spin contaminated, even though the RHF reference function is spin adapted. ump2 is not generally recommended, since spin contamination can lead to large errors.

Core orbitals can be specified as for the closed-shell methods. The number of electrons and occupations as well as the orbitals are taken from the most recent RHF calculation. It is possible to modify these defaults using occ, closed, wf, or orbital directives; see the MOLPRO reference manual for details.

The following is an example of a complete CCSD(T) calculation for O$_2$.

geometry={                 !geometry specification, using z-matrix

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

rccsd(t)                   !perform partially spin adapted CCSD(T) 2017-12-13