[gpaw-users] energy discontinuity with fixed magnetic moment

[Jason Gruber]

Hi all,

This issue came up in my work and I couldn't find any previous posts on this
topic, so here goes.  I've been doing some calculations with bulk metallic
solids at large lattice spacings, some of which tend to favor having a
non-zero magnetic moment, hence I've been using spin polarized
calculations.  What I find is that as I use smaller atomic volumes, the
converged magnet moment tends toward zero.

To check whether I'm converging to the right magnetic moment, I've performed
calculations where I set a fixed atomic volume and compute total energies as
a function of magnetic moment using fixmagmom=True.  In some cases, the
total energy and magnetic moment change smoothly until the moment converges
to zero, at which point I get a discontinuity in the energy.  Wondering why
this is, I'm asking

1.  When the magnetic moment is fixed to exactly zero with fixmagmom=True,
does GPAW switch to spin-paired mode?  I still get the magnetic moment
column of output during SCF either way, but, the energy computed this way
matches the energy computed with a spin-paired calculation.

2.  Is there a simple reason why the energy at zero magnetic moment jumps
discontinuously above the smooth curve I get with 0.005, 0.010, 0.015, etc.
fixed magnetic moments?  I'm using the PBE functional, but I don't see
anything in PBE that suggests the limit of the spin-polarized XC functional
doesn't converge to the spin-paired value.  Is it just that having the
additional eigenstates allows for more "variational freedom" to find lower
energies despite the fact that there are all paired electrons?

Thanks for any advice,

Jason
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