Is it possible to specifiy the multiplicity of a metallic (or bimetallic) system AND include the Fermi keyword?
Marcella Iannuzzi
marci... at gmail.com
Tue Nov 10 13:39:45 UTC 2015
Dear Natalie,
Ideally it should be always possible that the system converges to the
lowest energy state, whereas by fixing the multiplicity the system is
forced and keep the assigned number of electrons for the two spins up and
down.
On the other hand, to estimate energy differences among different spin
states, one can fix the multiplicity and try to prepare an initial guess as
close as possible to the desired spin state. If that spin state is at least
a metastable state, the wave function should converge there.
As it should be, cp2k gives both the possibilities. By applying the
Fermi-Dirac smearing the occupation of the two spin channels is adjusted in
order to reach the lowest energy state. By fixing the multiplicity, the
system is forced in a given state, and most probably, the Fermi energy is
going to be different for the two spin channels. For a non metallic system
this translates to having a different HOMO energy for spin up and spin
down.
Which are the stable states for your system is a problem not really related
to cp2k itself, but more to the level of theory and the other
approximations of the electronic structure calculations. Are you sure that
DFT, or the selected functional, or the basis sets and PPs are adequate to
describe the magnetic properties you are interested in?
If the state you are searching is not a minimum for the model you are
using, it will be difficult to converge there.
Anyway, the Mulliken population analysis is just a rough evaluation of the
charge and spin distribution. I would not take the Mulliken values too
strictly and I would use also other analysis tools.
>From a more technical point of view, the initial guess obtained by setting
the multiplicity and also by the broken symmetry approach can be useful to
bias the convergence toward a given state. However, starting from an
electronic configuration that is far from any stable state might cause
lengthy optimisation procedure. It is also possible that the SCF converges
to some wrong result, if it does not find its way back to a reasonable
minimum.
kind regards,
Marcella
On Monday, November 9, 2015 at 3:17:59 PM UTC+1, Natalie Austin wrote:
>
> Hello,
>
> I was able to get FIXED_MAGNETIC_MOMENT to work for an isolated Cu55
> cluster. It turns out that there isn't a significant difference in the
> total energy if I use this keyword or not. However when I incorporated a
> nickel atom into the cluster (Cu54Ni), the optimization process was really
> slow (3 steps in 31 hours). It seems that fixing the magnetic moment in
> this case is not probable.
> So my question still stands, is setting the multiplicity important if it
> is not reflected in the final result when using the fermi keyword?
>
> Any help with this would be much appreciated
>
> Thanks,
> Natalie
>
>
>
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