def simple_rmsd(s1,s2):
    """
    rmsd = sqrt((1/N)*sum((x_n - y_n)^2))
    """
    N = len(s1)
    _range = list(range(N))
    deviations = [(s1[i][0] - s2[i][0])**2 +
                  (s1[i][1] - s2[i][1])**2 +
                  (s1[i][2] - s2[i][2])**2 for i in _range]
    return ((1/float(N))*sum(deviations))**0.5