A one-electron quantum capping potential (QCP) for replacing the oxygen atom in silicate-type systems is developed. It is used to border the quantum region in a hybrid quantum mechanics/molecular mechanics (QM/MM) study. Its parametrization has been verified comparing the structures and atomic populations of silicate molecules with those provided by full QM calculations. The method has also been tested for the structure and (29) Si NMR shieldings of a mazzite cluster bordered with oxygen QCPs. (c) 2008 Elsevier B. V. All rights reserved.