Many metal-based antitumor drugs such as cis-diamminedichloroplatinum(II) (cisplatin) bind to purine nucleic acid (NA) bases via N7. On the basis of ab initio quantum mechanics calculations using [Pt(NH3)(3) purine](2+) (where purine = adenine or guanine) as a model system, we show that the common assumption of planarity of NA bases leads to an overestimation of the platinum out-of -plane bending molecular mechanics constant in certain cases by more than 50%. New values of force-field parameters are given, and some of the implications on molecular mechanics computations of platinum nucleic acid adducts are discussed.