A multidimensional surface used in the theory of electronic states of polyatomic molecules and chemical reactions. A potential-energy curve is the simplest type of potential-energy surface. Each internuclear distance in the molecule or reacting system is represented by one dimension, as is the potential energy. This means that if a nonlinear molecule has N atoms its potential-energy surface is a (3N – 6)-dimensional surface in a (3N – 5)-dimensional space. In the case of a linear molecule the potential-energy surface is a (3N – 5)-dimensional space. If the surface has a minimum then that electronic state is stable. It is possible for there to be more than one minimum. In potential-energy surfaces for chemical reactions the reactants and products are frequently ‘valleys’ connected by a region of higher energy, which is associated with the activation energy of the reaction.