def lj(r, epsilon, sigma, R):
    """Return lennard jones force and potential."""
    F = 4 * epsilon / (r-R) * (12 * (sigma / (r-R))**12 - 6 * (sigma / (r-R))**6)
    V = 4 * epsilon * ((sigma / (r-R))**12 - (sigma / (r-R))**6)
    return(V, F)