def _calculateForce(pot, r, h=1e-05):
    """Calls pot.force for separation (r) and returns DL_POLY -r dU/dr values rather than
  the dU/dr value normally returned by potentials.Potential.force() method

  @param pot potential from which force should be calculated
  @param r Separation at which force should be calculated
  @param h Increment used when performing numerical differentiation to obtain force

  @return -r dU/dr"""
    dUdr = pot.force(r, h=h)
    return r * dUdr
