[Pw_forum] question on constraint magnetic moment calculation

[Pang Rui]

Dear all
Could anyone gave me some suggestion to do the constraint magnetic moment
calculation?
I followed the constraint magnetic moment calculation of the following
handson with PWSCF.
http://www.vasp.at/vasp-workshop/slides/handsonIV.pdf
VASP uses the same method of PWSCF on constraint MM. However, I found the
constraint energy is difficult to go zero. In the handson, they use
lambda=50eV, got a constraint energy of 0.22591E-03eV. I used lambda=150Ry,
but got a constraint energy=0.35Ry. Even PWSCF is different with VASP on
some detail, I think it is not reasonable for two codes showing so large
difference. So could anyone point the unreasonable part of my input file?
Or give me some suggestion? This puzzled me for quite a long time.
The following is the input file.
&system
    ibrav=  0, nat=  2, ntyp= 2,
    ecutwfc = 80.0,
    occupations='smearing', 
    degauss=0.001,
    smearing='gauss' 
    starting_magnetization(1)=2.5
    starting_magnetization(2)=2.5
    angle1(1)=0.0
    angle1(2)=45.0
    angle2(1)=0.0
    angle2(2)=90.0
    constrained_magnetization="atomic direction"
    noncolin=.ture.
    lambda=150.0
    nosym=.true.
/
 &electrons
    mixing_beta = 0.1
    electron_maxstep=200
    startingwfc='file'
    startingpot='file'
 /
ATOMIC_SPECIES
 Fe1 56 Fe.pbe-spn-kjpaw_psl.0.2.1.UPF
 Fe2 56 Fe.pbe-spn-kjpaw_psl.0.2.1.UPF
ATOMIC_POSITIONS angstrom
Fe1 3.0 0.0 0.0
Fe2 5.0 0.0 0.0
K_POINTS automatic
1 1 1 0 0 0
CELL_PARAMETERS angstrom
    8.0    0.0000000000000000    0.0000000000000000
    0.0 8.0    0.0000000000000000
    0.0 0.0 8.0

PS1: I found the PWSCF used starting_magnetization as magnetization/charge
for generating initial wave function,but used it as magnetization in
constraint MM. So I set 2.5 for them.
PS2: The startingwfc is of a smaller lambda calculation. 
-- 
PostDoc
Department of Physics, South University of Science and Technology of China