We have studied the structure relaxation at the abrupt (110) Ge/GaAs interface by applying quantum chemical methods to clusters modeling this interface. Application of this model to bulk Ge and to bulk GaAs leads to theoretical Ge–Ge and Ga–As bond distances of 2.452 and 2.451 Å, respectively, in good agreement with the experimental values of 2.450 and 2.448 Å, respectively. Application of the model to the Ge/GaAs (110) interface indicates that this interface is nearly ideal. We find a very slight reconstruction at the interface leading to a Ge–Ga bond distance which is 0.04 Å longer than the Ge–As bond distance of 2.430 Å. The calculated spacing of the interface layer is 2.3% greater than that of bulk Ge or bulk GaAs.