where we have dropped the subscript (1,2 in Equation ) on µ, since this finite difference operation is valid at any state. µ j+1 is the dipole moment at +∆R and µ j-1 is the dipole moment at -∆R from the equilibrium positions. We also explicitly denote the derivative with respect to R as dµ/dR (i.e. without the boldface typography).