Here r {\displaystyle r} is the distance between the atoms, r e {\displaystyle r_{e}} is the equilibrium bond distance, D e {\displaystyle D_{e}} is the well depth (defined relative to the dissociated atoms), and a {\displaystyle a} controls the 'width' of the potential (the smaller a {\displaystyle a} is, the larger the well). The dissociation energy of the bond can be calculated by subtracting the zero point energy E 0 {\displaystyle E_{0}} from the depth of the well. The force constant (stiffness) of the bond can be found by Taylor expansion of V ′ ( r ) {\displaystyle V'(r)} around r = r e {\displaystyle r=r_{e}} to the second derivative of the potential energy function, from which it can be shown that the parameter, a {\displaystyle a} , is