Calculates the quasiparticle weight in single
       site spin hamiltonian under with N degenerate half-filled orbitals

def follow_cf(save, Uspan, target_cf, nup, n_tot=5.0, slsp=None):
    """Calculates the quasiparticle weight in single
       site spin hamiltonian under with N degenerate half-filled orbitals """

    if slsp == None:
        slsp = Spinon(slaves=6, orbitals=3, avg_particles=n_tot,
                       hopping=[0.5]*6, populations = np.asarray([n_tot]*6)/6)

    zet, lam, mu, mean_f = [], [], [], []

    for co in Uspan:
        print('U=', co, 'del=', target_cf)
        res=root(targetpop, nup[-1],(co,target_cf,slsp, n_tot))
        print(res.x)
        if res.x>nup[-1]: break

        nup.append(res.x)
        slsp.param['populations']=population_distri(nup[-1])
        mean_f.append(slsp.mean_field())
        zet.append(slsp.quasiparticle_weight())
        lam.append(slsp.param['lambda'])
        mu.append(orbital_energies(slsp.param, zet[-1]))

#    plt.plot(np.asarray(zet)[:,0], label='d={}, zl'.format(str(target_cf)))
#    plt.plot(np.asarray(zet)[:,5], label='d={}, zh'.format(str(target_cf)))

    case = save.createGroup('cf={}'.format(target_cf))
    varis = st.setgroup(case)
    st.storegroup(varis, Uspan[:len(zet)], zet, lam, mu, nup[1:],target_cf,mean_f)