[Pw_forum] EXX routine

[Nicola Marzari]

Helen wrote:
> This method works fine for LDA routines. It also works OK with the 
> functional we made (BNL, which is a type of hybrid functional of LDA and 
> explicit exchange) when the weight given to the explicit exchange is 
> small. However when I use only the explicit exchange routine or the EXX 
> routine has a more significant weight than LDA, I get spurious results 
> for the general k-point.


I'll indeed let Stefano chip in, or any other - one question that comes
to mind is this: would the results change if you had a much tighter
tolerance for convergence ?

The code has "global" tolerance thresholds, to which the quasi-empty
k-points do not contribute. I think the Davidson algorithm should
diagonalize equally well the quasi-empty, but if it struggles, the code
will not recognize their struggle.

Another naive comment: couldn't you use the nscf or bands option,
to diagonalize a posteriori the Hamiltonian at any arbitary point ?

A few more points: I am not sure with which precision the
code reads the weights (maybe your small weights are read as
zero) - and what it does if the sum of the weights
is not normalized to 1.0 or 2.0 (as in your case).

Last sanity check: what happens if you also give k=0,0,0
a second time, but with the almost-zero weight ?

All the best,

				nicola

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