Ab initio configuration interaction calculations of the hyperfine structure in small radicals

Extended basis set natural orbital CI wave functions have been used to compute the isotropic and anisotropic hyperfine coupling of ten small radicals for which experimental measurements exist. The configuration spaces employed in the CI’s were of two kinds: One involved all single and double excitations from the Hartree–Fock configuration plus limited triples and quadrupoles, while the other consisted of singles and doubles from a multireference set coupled with various configuration selection techniques. At the highest level of theory used in the present study, agreement with experiment was generally within 10%, but differences for the isoelectronic series H2CN, H2CO+, and H2BO as well as the related acetaldehyde radical cation H3C–CHO+ are larger than would be expected based on the data from the other six radicals.