[CP2K-user] [CP2K:19315] Re: Is it possible to specifiy the multiplicity of a metallic (or bimetallic) system AND include the Fermi keyword?

[Moon Yue]

Dear Iannuzzi,

Thank you for your comments.
I have a question. In order to set the total magnetic moments as any 
values, are the function  keyword MULTICLICITY and  FIXED_MAGNETIC_MOMENT 
equal ? If not, could you please tell me the difference ? Thank you.

Yue Qiang

在2015年11月10日星期二 UTC+9 22:39:45<Marcella Iannuzzi> 写道：

>
>   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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