[Pw_forum] Problem with nbnd in clusters

[Cyrille Barreteau]

I just wanted to add that changing the diagonalization
scheme to conjugate gradient 'cg' instead of the
default 'david' seems to solve the problem in my case.
I now find the same result with nbnd=12,20,25..

Does anyone has experience about the behaviour
of the various diagonalization schemes?
Which one is better, in which case?

   cyrille

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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