Localized molecular second-order properties

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Abstract
Some second-order properties like the electrical polarizability, magnetic susceptibility and proton magnetic shielding, computed by the coupled-perturbed SCF method for H2O NH3 CH4 and CH3F were analysed in terms of localized orbital contributions. For the magnetic observables the gauge transformation introduces some ambiguities in the definition of orbital contributions, besides the well-known difficulties of yielding results which are not gauge invariant for limited bases. These problems are examined in detail and a method is proposed to overcome the effect of the limitation of the basis. Conditions are also stated under which the ambiguities may be resolved. By assuming that the localized orbital contribution may be transferred from one molecule to another, estimates are made for second-order properties of CH2F2, CHF3 and CF4 molecules. The agreement of these estimates with the available experimental data is better than expected.