Is it possible to specifiy the multiplicity of a metallic (or bimetallic) system AND include the Fermi keyword?

[Marcella Iannuzzi]

  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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