[Pw_forum] fixed spin moment calculation does not give a correct lattice constant !?

[xwzhang]

Dear All,

When I am trying to do FSM calculations to fcc-Fe, I met such strange things:
The equilibrium volume of magnetic fcc-Fe is about 72 (au)^3, and nonmagnetic one about 70 (au)^3(PRB,60, 3839).
When I fixed the magnetic moment, the total energy decreases with the increase of the volume over all the volume range I used: from 50 (au)^3 to 120(au)^3. One of the input is as follows: (in other calculations I just vary the celldm(1) )
 &CONTROL
                       title = fcc-fe ,
                 calculation = 'scf' ,
                restart_mode = 'from_scratch' ,
                      outdir = './' ,
                  pseudo_dir = './' ,
                  wf_collect = .true.,
                     tstress = .true.,
                      prefix = pwscf,
                     tstress = .false. ,
 /
 &SYSTEM
                       ibrav = 2,
                   celldm(1) = 6.60,
                         nat = 1,
                        ntyp = 1,
                     ecutwfc = 30 ,
                     degauss = 0.01,
                     ecutrho = 350,
                    smearing = 'mp',
                 occupations = 'smearing',
                       nspin = 2,
   constrained_magnetization = 'total',
      fixed_magnetization(3) = 0.0
 /
 &ELECTRONS
 /
 &IONS
 /
 &CELL
 /
ATOMIC_SPECIES
   Fe   55.84700  Fe.pw91-sp-van_ak.UPF
ATOMIC_POSITIONS alat
   Fe      0.000000000    0.000000000    0.000000000
K_POINTS automatic
  20 20 20   1 1 1
  
I do not know the reason. And the total energy is twice as the nonmagnetic calculation (nspin=1) . In my mind they should be the same if I set total moment to 0.0

Regards and happy weekend!

Dr. Wenxu Zhang

State Key Laboratory of Electronic Thin Films and Integrated Devices,
University of Electronic Science and Technology of China,
Chengdu 610054
China
Tel: +86-28-83201475
Fax: +86-28-83204938
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