[Pw_forum] implementation of variation of occupations and Fermi level with perturbations

[David Strubbe]

Paolo,

Thanks for the response. This gives me some more insight into what is going
on, but I still don't understand.

As far as I can tell, the drhoscf in ef_shift is still the change in the
density rather than the change in the potential, because the calls in
solve_linter to dv_of_drho are using a copy of the density response rather
than drhoscf itself:

        call zcopy
(nrxx*nspin_mag,drhoscfh(1,1,ipert),1,dvscfout(1,1,ipert),1)
        call dv_of_drho (imode0+ipert, dvscfout(1,1,ipert), .true.)

The heart of the matter is the lines in ef_shift:

delta_n = delta_n + omega*drhoscf(nl(1),is,ipert)
def (ipert) = - delta_n / dos_ef

delta_n does not seem to refer to \Delta n_{ext} or the integral of the LDOS
with \Delta V_{SCF}, as in the numerator of Eq. 79. Do those quantities
exist in the calculation here?

Thanks,
David Strubbe
UC Berkeley

On Tue, Apr 19, 2011 at 1:44 PM, Paolo Giannozzi <giannozz at democritos.it>wrote:

>
> On Apr 8, 2011, at 20:39 , David Strubbe wrote:
>
> > Eq. 79 refers to a quantity \Delta n_{ext} and an integral of the LDOS
> > with \Delta V_{SCF} to calculate the shift in Fermi level. However in
> > ef_shift it appears that the density response drhoscf is used instead
> > of these quantities in the numerator, which doesn't seem like the
> > same thing.
>
> it doesn't, but it is. The (dirty) trick is well hidden in the call
> to routine dv_of_drho:
> it accepts in input the variation of the charge density, returns in
> output the
> variation of the potential, overwritten on the former. It was done a
> looong time
> ago in order to spare some memory, when machines had much less ram and
> the code was much simpler and smaller (occasional dirty tricks were
> under
> control, sort of). A comment in the code would have spared you (and me)
> some time. Unfortunately comments in code tend to belong to one of the
> following categories: 1) useless, 2) misleading, 3) obsolete, 4)
> nonexistent
>
> P.
> ---
> Paolo Giannozzi, Dept of Chemistry&Physics&Environment
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
>
>
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