A multireference CI determination of the isotropic hyperfine constants for first row atoms B–F

Abstract

The requirements for obtaining converged isotropic hyperfine constants, Aiso, in first row elements B–F are investigated with extended basis set CI wave functions composed of single and double excitations from a multiconfiguration reference space. Since the restricted Hartree–Fock unpaired spin density at the nucleus is zero for these elements, correlation effects account for the entire answer. Although the wave functions computed in the present study uniformly recovered 94%–98% of the empirical correlation energy, agreement with gas phase measurements of Aiso varied from a low of 55% on B to a high of 96% on N. Extension of these findings to the B2 (3Σg ) and H2 CO+ (2B2 ) molecules is also discussed. In order for small basis sets or very lightly correlated wave functions to produce good agreement with experiment, fortuitous cancellation of error between the negative core and positive valence contributions must occur. The present findings suggest relatively large basis sets and substantial correlation recovery are necessary if reliable core/valence balance is to be achieved.

The hyperfine coupling constants of 19F: An ab initio MRD‐CI basis set study

“…Systematic studies of the basis set dependence of also for other systems such as I4N using MRSD-CI wave functions were reported by Engels et al [7] and Feller and Davidson [8]. For "F,, two theoretical studies are available: one by Nguyen and Ha [9] and the other by Carmichael [lo].…”

Hyperfine Coupling Constants from Internally Contracted Multireference Perturbation Theory

“…The methods based on the complete active space selfconsistent field (CASSCF) and (uncontracted) multiconfiguration interaction (MRCI) methods have been reported as well. [13][14][15][16] Yet another approach is based on density functional theory (DFT). 10,[17][18][19][20][21] These approaches have been generalized to relativistic analogues to account for the scalar relativistic and spin-orbit effects.…”

Full configuration interaction and multiconfigurational spin density in boron and carbon atoms

“…For a nitrogen atom, which is a much simpler system with respect to spin density calculations than boron or carbon, 19,27 it was demonstrated that at least quadruple excitations are required if a single reference wave function is used. A multireference CI wave function based on a relatively small CASSCF reference function predicts a N spin density that is very close to FCI.…”