THE MC-IGLO METHOD

The MC-IGLO method for the calculation of NMR chemical shifts and magnetic susceptibilities with inclusion of electron correlation effects is presented. It is a generalization of the IGLO (individual gauge for localized orbitals) approach to multiconfiguration reference functions, and is especially suited for molecules that are not well described by a single Slater determinant like ozone, B4H4, cyclobutadiene and generally molecules with both lone pairs and double bonds. In those cases where the conventional IGLO method (SCF-IGLO) - like all comparable methods - fails, MC-IGLO improves the results considerably, although the correlation effects sometimes overshoot. For closed-shell systems like H2O and CH4 MC-IGLO results differ little from SCF-IGLO, and to obtain higher accuracy in these cases, the extension of the basis is more important than the inclusion of correlation effects. Explicit results are presented for H2O, N2, PN, NNO, SO2, B4H4 and finally cyclobutadiene C4H4 for geometry ranges between the rectangular equilibrium structure and a square form.