THE MC-IGLO METHOD

The MC-IGLO method for the calculation of NMR chemical shifts and magn etic susceptibilities with inclusion of electron correlation effects i s 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 b y a single Slater determinant like ozone, B4H4, cyclobutadiene and gen erally molecules with both lone pairs and double bonds. In those cases where the conventional IGLO method (SCF-IGLO) - like all comparable m ethods - fails, MC-IGLO improves the results considerably, although th e correlation effects sometimes overshoot. For closed-shell systems li ke H2O and CH4 MC-IGLO results differ little from SCF-IGLO, and to obt ain 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 cyclobutadi ene C4H4 for geometry ranges between the rectangular equilibrium struc ture and a square form.