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The MP2 and all CC calculations for ROHF reference wavefunctions are
done by first transforming to a semi-canonical orbital basis which are
defined by the eigenvectors of the occupied/occupied and
virtual/virtual blocks of the Fock matrices of alpha and beta spin.
No spin restrictions are applied in the cluster equations. This
approach is sometimes also denoted as ROHF-UCCSD.
Note that if a frozen-core approximation is used, the semicanonical
orbitals depend on whether the block-diagonalization of the Fock matrices
is done in space of all orbitals or only in the space of the correlated
valence orbitals. The two approaches lead thus to slightly different
energies, but none of two is more valid or more accurate than the other.
The `ricc2` program uses the former scheme with the block-diagonalization
done in the space of all molecular orbitals.
The same scheme is used e.g. in the CFOUR program suite, but other
codes as e.g. the implementation in MOLPRO use a block-diagonalization
restricted to the active valence space.

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TURBOMOLE M