The application of quantum chemical calculations of the all-electron ab initio type studies of metal influence on neighbouring hydrogen bonds has been extended successively to new and larger model compounds during the past decade. Starting with comparatively small systems accessible to fair methodical accuracy due to the use of larger basis sets, some principal effects have been revealed, such as bond contraction, net bond stabilization, charge transfer effects and the increase of rotational barriers around the hydrogen bond [1, 2]. At the same time, such investigations have proved once more the need for the use of ab initio type calculations and the incapability of semiempirical procedures for the treatment of metal-solvent or metal-ligand interactions. The consideration of slightly larger systems such as e.g. ion/water/formamide [3] has shown that the effect of the metal ion extends over a long series of bonds, which implies a need for comparatively large ‘subunits’ of the molecules (or rather ‘supermolecular’) system for a conclusive quantum theoretical study. This fact was also revealed by the quantum chemically predicted electronic rearrangements taking place in complexes with more than one ligand with the ability to form chelate structures [4]. For this reason, the applicability of minimal basis sets, allowing an extension of the supermolecular model system to considerably larger subunits, had to be tested. The conclusions of such test [2, 3] have shown that such basis sets can be used for the evaluation of intermolecular geometrical parameters and relative energy effects, still maintaining sufficient accuracy, especially if basis set error corrections like the counterpoise procedure [5] are employed. The data of ab initio calculations for ion/ligand interaction form the basis of three main approaches to more complex systems: 1. The evaluation of pair and three-body potentials representing the basis for the construction of interaction potentials used in methods like Monte Carlo of Molecular Dynamics calculations on large ensembles of ions and molecules and/or macromolecules [ e.g. 6–8]. 2. The ‘static’ treatment of large supermolecular species with small basis sets for the evaluation of geometries, binding energies, electron density distribution and other data for solvated ions or ion/ligand complex. 3. The possibility of the construction of supermolecular electrostatic perturbation fields representing the influence of molecules at larger distances from the ion and/or directly involved ligand [9,10]. Among these methods, numbers 2 and 3 are especially suitable to reveal some effects of the metal ions on surrounding solvent and other ligand molecules. For the demonstration of the use of small basis sets in combination with electrostatic potentials, solvation of ions in water and some nonaqueous solvents are mentioned [9, 10]. The treatment of crystal growth also seems to be a promising field of application for this approach [11]. The application of calculations of the type 2 mentioned above for the study of metal ion influence on hydrogen bonds in base pairs of nucleic acids [12–14] seems to be another useful example of the capability of theoretical approaches in the field of bioinorganic chemistry. These examples also demonstrate, however, the importance of basis set error corrections and hence a careful methodical control in the course of such investigations. An example of cooperative experimental and quantum chemical research can be given in the field of ion/peptide interactions, where experimental data for biological protein/metal complexes are compared with metal complexes of small peptides, which on the other hand are accessible again to ab initio calculations with minimal basis set.