Abstract
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, B 4H 4, 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 H 2O and CH 4 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 H 2O, N 2, PN, NNO, SO 2, B 4H 4 and finally cyclobutadiene C 4H 4 for geometry ranges between the rectangular equilibrium structure and a square form.
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