BASIS-SETS AND ACTIVE SPACE IN MULTICONFIGURATIONAL SELF-CONSISTENT-FIELD CALCULATIONS OF NUCLEAR-MAGNETIC-RESONANCE SPIN-SPIN COUPLING-CONSTANTS

The dependence of spin-spin nuclear magnetic resonance (NMR) coupling constants on the basis set and electron correlation has been investiga ted in methane using Hartree-Fock and multiconfigurational self-consis tent field wave functions (HF-SCF and MCSCF). The effect of the size, contraction, and tight s functions of the basis sets is analyzed. Some suggestions about the contraction scheme are indicated. MCSCF wave fu nctions with different numbers of active orbitals and different number s of excited electrons were used. An approximation to determine spin-s pin coupling constants at a high level of electron correlation from th ree calculations with a smaller level of correlation and reduced compu tational cost is investigated. The best calculated (1)J(CH) and (2)J(H H) couplings are 120.63 and -13.23 Hz, respectively, which are 0.24 an d 1.24 Hz smaller than those experimentally obtained for the equilibri um geometry. The remaining error in these coupling constants can be at tributed mainly to correlation and not to basis set effects. (C) 1998 American Institute of Physics. [S0021-9606(98)31043-0].