The equilibrium geometry of any polyatomic system in nondegenerate states is defined as corresponding to the point of the minimum of the adiabatic potential energy surface (APES), where its first derivatives are zero and the second derivatives are positive. If we denote the energy of the system as a function of normal displacements Q α α as E ( Q α α ) , at the minimum point of the APES ( Q α α = 0 ), the curvature K of E ( Q α α ) in direction Q ,