1. The problem statement, all variables and given/known data If there are a large number of ions oscillating in a straight line, we can pick the nth one oscillating about its equilibrium a*n. The potential of the entire lattice is then U = 0.5*K[u(an)-u([n+1]a)]^2 - summed over all n. How do I use Force = -dU/du(an) to derive that Force = -K[2u(an)-u([n-1]a)-u([n+1]a)? Where K is some constant. 2. Relevant equations The chain rule is f'(g(x))= df/dg * dg/dx 3. The attempt at a solution I tried using the chain rule. Then if g = [u(an)-u([n+1]a)], then dU/dg = k*[u(an)-u([n+1]a)]. dg/du(an) can't be done analytically, however because we have a set of discrete points, dg = [(u(an) - (u[n-1]a) - (u[n+1]a)-u(an))]. I get dU/du(an) = K*[u(an)-u([n+1]a)]*[2u(an)-u([n-1]a)-u([n+1]a)]/du(an). I am guessing I could write that du = [u([n+1]a)-u(an)] to get dU/du(an) = -K*[2u(an)-u([n-1]a)]u([n+1]a)], but there's a minus sign that I can't get rid of. Am I going about this all wrong?