Problem in Spin Polarized Calculation Using CASTEP All BAND/EDFT

Dear All

I have a problem of spin polarized calculation using CASTEP all band/EDFT method.

My system is a 4-layer 2x2 Rh(111) surface. The Spin polarized calculation is supposed to give an optimized structure without spin on Rh atom. However, the spin polarized calculation shows spin at individual Rh atom, even though the total spin is zero.

Then I tried spin unpolarized calculation. It gave a different total energy, which is 2 eV higher than that of the spin polarized calculation. But as Rh does not have magnetism, the spin polarized and unpolarized calculations are supposed to give same results.

BTW, I just updated the MS a few days ago.

Here is my input for the Spin Polarized calculation of Rh(111)

task : GeometryOptimization

xc_functional : PW91

spin_polarized : true

spin :       0

opt_strategy : Default

page_wvfns :       0

cut_off_energy :     400.000000000000000

grid_scale :       1.750000000000000

fine_grid_scale :       1.750000000000000

finite_basis_corr :       0

elec_energy_tol :   1.000000000000000e-006

max_scf_cycles :     1000

fix_occupancy : false

metals_method : EDFT

num_occ_cycles : 6

nextra_bands :       18

smearing_width :       0.100000000000000

spin_fix :       6

geom_energy_tol :   2.000000000000000e-005

geom_force_tol :       0.050000000000000

geom_stress_tol :       0.100000000000000

geom_disp_tol :       0.002000000000000

geom_max_iter :     1000

geom_method : BFGS

fixed_npw : false

calculate_ELF : false

calculate_stress : false

popn_calculate : true

calculate_hirshfeld : true

calculate_densdiff : false

popn_bond_cutoff :       3.000000000000000

pdos_calculate_weights : true

num_dump_cycles : 0

The output gives the spin on individual Rh:

Rh       1     -0.80

Rh       2     0.80

Rh       3     0.68

Rh       4     -0.68

Rh       5     -0.80

Rh       6     0.80

Rh       7     0.68

Rh       8     -0.68

Rh       9     -0.80

Rh     10     0.80

Rh     11     0.68

Rh     12     -0.68

Rh     13     -0.80

Rh     14     0.80

Rh     15     0.68

Rh     16     -0.68

Well, the spin is supposed to be nearly 0, like 0.02 something.

Does any one knows the reason?

Thanks a lot