Electronic structure investigations on triphenylphosphine (PPh3) ligated gold clusters are commonly carried out with model phosphine ligands. To explore the validity and the limitations of this approach, we have studied the effects of phosphine substituents in a series of gold(1) compounds: MeAuPR3, R = H, Me, Ph. We used the recently developed scalar-relativistic version of the linear combination of Gaussian-type orbitals (LCGTO) local density functional (LDF) method which allows an all-electron treatment of all systems under study. For structural properties the PH3 ligand provides a satisfactory model of the full PPh3 ligand. But the trimethylphosphine ligated models have to be employed if good agreement is desired for energy properties and for the dipole moment.
An efficient quantum chemical method for calculating dispersion energy shifts in molecular electronic spectra is described. The method makes use of separate calculations for solvent and solute molecules using a perturbation theory formula which averages over solvent configurations. To maintain the balance between the dispersion depression of the ground state and that of an excited state in a finite configuration interaction (CI) treatment, the Thomas-Reiche-Kuhn sum rule is invoked twice, correcting for the missing higher excited and continuum states in both the solute and the solvent. The resulting modified perturbation expression for the dispersion energy is remarkably stable with respect to the size of the employed CI. The model is implemented and applied in the framework of the intermediate neglect of diatomic overlap method using a previously presented selfconsistent reaction field description for the solvent and the solute molecules. Applications to acetone, benzene, naphthalene, and chrysene in cyclohexane clearly demonstrate the success of the method.
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