[QE-users] constrained magnetization with non-colin and spin-orbit

[Matteo Cococcioni]

Dear all,

I am trying to run some calculations on a molecule with two magnetic ions
(Mo) and I want to use the non collinear spin with spin-orbit to calculate
the energy needed to change their direction. Starting from a collinear
(lsda) calculation I managed to converge the non-collinear one in a
ferromagnetic configuration, with magnetic moments aligned along z. Now I
am using this ground state as starting point for a calculation where one or
both spins are somehow deviated from the z direction. Following the
instructions in INPUT_PW I am using the following settings (in &system):

    noncolin = .true.
    lspinorb=.true.
    angle1(1) = 30.0
    angle1(2) = 30.0
    constrained_magnetization = 'atomic direction'
    lambda = 1.0

where species 1 and 2 correspond to the two Mo, angle1 is the angle I want
to have between z and the final magnetization, lambda is the strength of
the quadratic constraint.

If I start from the potential of the ground state with the magnetization
along z, the code starts with no problem and even pretends to converge for
a number of iterations. Then suddenly the energy explodes and the code
crashes saying that there are too many not converged eigenvalues.
While it seems converging nothing relevant seems to happen to angles: the
constraint energy remains almost the same and the magnetization of both
atoms maintains its original direction (aside small fluctuations). The same
behavior is observed independently from the value of lambda (which is very
strange) and beta (the mixing parameter).

looking into the code (add_bfield.f90) the implementation of the constraint
seems fine, as far as I can tell (except that the code tries to constraint
all magnetic moments once a lambda is present, which I fixed), and the
potential seems to get a term from the constraint on magnetization. So I
would expect it to do something.

Does anyone have any experience with this type of calculations? What am I
missing or not doing right? Thanks in advance for any help/advice.

Best regards,

Matteo

-- 
Matteo Cococcioni
Department of Physics
University of Pavia
Via Bassi 6, I-27100 Pavia, Italy
tel +39-0382-987485
e-mail matteo.cococcioni at unipv.it <lucio.andreani at unipv.it>
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