[Pw_forum] Problem with nbnd in clusters

[Nicola Marzari]

Hi Cyrille,

your tm dimer will likely require spin polarization, and
will have several scf solutions that correspond to different
S_z (that you can fix in several ways, either specifying the
number of spin up and spin down, or using a fermi_up and
fermi_down energy, to specify n_up - n_down but still treating
the system with a smearing and fractional occupations).

Even for a given S_z, there will be different scf solutions
that have different spatial symmetry.

So, in your case I would definitely switch spin-polarization,
and maybe play around with small and large smearings, to see what
happens, and look at the states, and the occupations. Anyhow, if
you get to a scf solution, that is a "good" solution for the GGA
problem, although might not be very physical.

Also, have a look at Kulik et al PRL 2006.
	
			nicola


Cyrille Barreteau wrote:
> Dear pwscf_ers
> 
> I am doing a very simple calculation on a transition metal
> dimer and I have a encoutered a strange problem (I love
> strange problems:-)
> 
> I have done a first calculation with the default value
> of nbnd, ie nbnd=nelec/2*20% (=12 in my case)
> 
> But since it is known that in clusters it is often good
> to increase nbnd I have performed another calculation with
> larger nbnd (=20). The result is quite different from
> the one at nbnd=12.
> 
> I have then increased again nbnd up to 25 and then I recover
> the result obtained for nbnd=12
> 
> More problematic is the local density of states (I know
> it is not really a dos but a bunch of dirac peaks).
> In a dimer with the z axis along the direction connecting
> the two atoms, the xy and x^2-y^2 dos should be degenerate
> (if the supercell box is large enough).
> In fact the xy and x^2-y^2 are degenerate if nbnd=12 or 25
> but there is a rather large splitting if nbnd=20.
> 
> I am quite sure the result for nbnd=20 is not correct but
> what is the origin of this problem?
> 
> Maybe I could try to use another diagonalization scheme..
> 
>    thanks for reading my strange problems
> 
>      cyrille
> 
> 


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Prof Nicola Marzari   Department of Materials Science and Engineering
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