Dear all, I'm trying to compute the dynamical matrix of a crystal with a pair potential. One way would be to use the second derivatives of the potential, and an other is to compute it numerically. Well, in both cases, I get troubles where atom i equal atom j. Is there something I haven't get yet? I go through each atoms (i=1,N) and for each pairs I go through each coordinates (x,y,z). But in the analytical expressions of the derevatives there is always 1/r which diverge when r=0. Numerically I face to the same problem .... Any advices? Thanks in advance, Eric.