Flow chart describing a pilot implementation of the spin-adapted CC approach within the CCSD approximation. For further details see text.
Comparison of various CC approaches to treat high-spin open-shell cases. Details concerning the considered projected amplitude and spin equations, and the cluster operator, as well as the preserved spin properties, are given for the specific case of the CCSD approximation. , , and denote the number of determinants, spin-adapted configurations and , respectively, in the excitation space, while denotes the number of configurations in the full space.
Correlation energies (in hartree) for open-shell molecules computed at the CCSD level with different treatments of spin. (Calculations have been performed at experimental geometries [ (Ref. 37), (Ref. 37), (Ref. 37), , (Ref. 38), , (Ref. 37), , (Ref. 37), , (Ref. 39), (Ref. 37), (Ref. 37)] using the cc-pVDZ basis (Ref. 40) and the frozen-core orbital approximation.)
Check of the size consistency of different open-shell CC schemes. Reported are the correlation energy of the subsystems and , as well as that of the noninteracting supersystem . The size-consistency error is defined as the energy difference of the latter and the former. The 3-21G basis (Ref. 42) has been used. All values are in hartree.
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