perform a better initial guess of lambda
       no improvement

def pressision_try(orbitals, U, beta, step):
    """perform a better initial guess of lambda
       no improvement"""
    mu, lam = main(orbitals, U, beta, step)
    mu2, lam2 = linspace(0, U*orbitals, step), zeros(step)
    for i in range(99):
        lam2[i+1] = fsolve(restriction, lam2[i], (mu2[i+1], orbitals, U, beta))
    plot(mu2, 2*orbitals*fermi_dist(-(mu2+lam2), beta), label='Test guess')
    legend(loc=0)