[Pw_forum] Problem with nbnd in clusters

[Cyrille Barreteau]

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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