EFFICIENT PREDICTION OF ISOTROPIC HYPERFINE COUPLING IN RADICALS CONTAINING PHOSPHORUS - A SYSTEMATIC COMPARISON OF UHF, PUHF, AND UMP2 SPIN-DENSITIES

Isotropic hyperfine couplings have been calculated and compared to exp eriment for 25 radicals containing phosphorus using the 6-311G* basis set. At HF/6-31G* optimized geometries, Fermi contact integrals deri ved from UMP2 spin-density matrices give the best correlation with exp eriment (20 data points for P-31, 8 data points for F-19, 7 data point s for Cl-35, and 5 data points for H-1 hyperfine couplings, rms error of 22.9 G with the data spanning a range of about 1650 G); PUHF spin-d ensity matrices are somewhat less useful (rms error of 33.6 G), and ca lculation at the UHF level gives poor results (rms error of 45.6 G). H yperfine couplings calculated at the UMP2 level may be scaled by 102% to bring them into closer agreement with experiment. Even though geome tric changes on going to the MP2 level may be sizable, better correlat ion with experiment is obtained when HF/6-31G* optimized geometries a re used instead of MP2/6-31G* optimized geometries (rms error of 36.1 G). Including diffuse functions in the calculation of the hyperfine c ouplings for anionic radicals has in general only marginal effect. Sta tistical trends within specific subsets of the data are also discussed .