An example showing the partitioning of the 1-particle Fock space into 3 sub-spaces for RAS-SF. The orbital occupations corresponding to the single configuration high spin reference are shown on the left (where all N strongly correlated electrons are spin-parallel; N = 2 in the figure). In the multiconfigurational RAS-SF wave function on the right, all orbital occupations of the active (RAS II) levels are permitted with a specified number of spin flips (typically n = N/2, or 1 in this example). Additionally, single excitations into RAS II from the hole space, and single excitations from RAS II into the particle space are allowed, as indicated schematically by the arrows.
Dissociation curve for lowest doublet ground state and the ROHF reference sextet state of the dinitrogen radical cation.
Natural orbitals from RAS-SF for the DMX triradical from the quartet reference and 5e, 5o active space.
Comparison of RAS-SF (with the high spin reference indicated in the legend) to EOM-SF-CCSD for DMX radical excited states. States 1 (2B2) and 3 (2A1), 4 (2A2), 5(2B1) are doublets at all levels of theory, while state 2 is 4B2. The character of each state matches those from EOM-SF-CCSD.47
The triradical doublet NTB-TOT and its 3 SOMOs are illustrated on the top row, while the diradical triplet NTB-TOT+ and its two high spin coupled SOMOs, are shown on the bottom row. The 3 unpaired electrons of the neutral antiferromagnetically couple to a doublet ground state. By contrast, after 1e oxidation of NTB-TOT to yield NTB-TOT+, the 2 remaining unpaired electrons ferromagnetically couple to a triplet ground state.
Radical conversions upon 1e oxidation for NTB-TOT, NTB-TPA, and NTB-TPM. RAS-SF spin state assignments agree with available experimental results.
Tp*Co(NO) complex and its natural orbitals from RAS-SF, where Tp* is hydro-tris(3,5-dimethylpyrazolyl)borate.
Tp*Ni(NO) complex and its natural orbitals from RAS-SF.
Natural orbitals and occupation numbers from RAS-SF for CoL.
CPU timings for RAS-SF/6-31G* calculations on the DMX triradical (C8H7; 134 basis functions), with 8 states and the frozen core approximation, on a 4 ×8 core AMD Opteron 6100 series node running at 2.3 GHz. For comparison, the quartet ROHF/6-31G* calculation required 18.1 s on 8 cores.
CPU timings for RAS(5,5)-1SF/6-31G* calculations on the NTB-TOT triradical (C30O6N4H33; 666 basis functions), with 8 states and the frozen core approximation, on a 4 × 8 core AMD Opteron 6100 series node running at 2.3 GHz. For comparison, the quartet ROHF/6-31G* calculation required 126 s on 8 cores.
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