Spin‐Adapted restricted Hartree–Fock reference coupled‐cluster theory for open‐shell systems: Noniterative triples for noncanonical orbitals

The coupled clusters singles and doubles ( CCSD ) method for calculations of open‐shell systems with the single restricted Hartree–Fock ( ROHF ) reference determinant is extended by the noniterative triples to give CCSD(T) . Our approach profits from the fact that (a) single‐ and double‐excitation amplitudes are spin‐adapted, which directly leads to a computationally less demanding algorithm than are nonadapted procedures and produces the spin‐adapted CCSD wave function and (b) triple excitations calculated from converged spin‐adapted ( SA ) CCSD amplitudes are also obtained more effectively. Altogether, computer demands of our SA CCSD(T) approach, applicable to high‐spin open‐shell cases which are well represented by a single‐determinant reference is comparable to that for closed‐shell systems. Our approach is not based on semicanonical orbitals, applied by Bartlett's group. However, we compare some other possible choices of ROHF orbitals to this “standard.” Numerical results for a series of atoms and molecules demonstrate little sensitivity to this selection.