Stefano, Thank you for the explanation. Things are clear now. Is there a circumstance in which Delta n_ext(q=0) is nonzero?
David Strubbe UC Berkeley On Wed, Apr 20, 2011 at 1:12 AM, Stefano de Gironcoli <degironc at sissa.it>wrote: > Dear David Strubbe, > > eq. 79 is a general formula; in the phonon code Delta n_ext(q=0) is always > vanishing because the perturbation is neutral (atoms are displaced but their > charge is not changed), still Delta Vscf is in general non zero so one has > to do something > > Let's look at the scf variation of rho that is something like > > Delta rho(r) = sum_i f_i (phi_i* Delta_phi_i +cc) + d f_i/de *(Delta e_i - > Delta e_F) |phi|^2 > > As said, eq. 79 is the condition of charge neutrality of the perturbation > which is due ONLY to the last term involving changes in occupation of states > (remember that Delta phi is always orthogonal to the unperturbed phi so in > no case Delta phi can contribute to the total charge variation) > > if you work out the expression ... > > total charge = Delta n_ext + Delta_n induced by Delta e_i (defined in terms > of the potential change) + Delta_n induced by the Fermi energy shift.. > > and the neutrality condition reads > > 0 = Delta_n_ext + \int n(e_F,r) Delta Vscf - n(e_F) * Delta e_F > > which is eq. 79. > > HOWEVER, delta rho that enters ef_shift has been computed without the > contribution coming from the (then unknown) Fermi energy shift Delta e_F > that is (see above) > > - n(e_F,r) * Delta e_F > > this is how ef_shift routine icomputes Delta e_F: it calculates the G=0 > component of Delta rho before the shift ( which amount to be \int n(e_F,r) > Delta Vscf ) and define Delta e_F in such a way that after the term - > n(e_F,r) * Delta e_F is added the final Delta rho is neutral. > > I hope the procedure is a bit clearer now. > > stefano > > > > On 04/20/2011 12:08 AM, David Strubbe wrote: > > 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> > <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 > > > > _______________________________________________ > Pw_forum mailing listPw_forum at > pwscf.orghttp://www.democritos.it/mailman/listinfo/pw_forum > > > _______________________________________________ > Pw_forum mailing listPw_forum at > pwscf.orghttp://www.democritos.it/mailman/listinfo/pw_forum > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20110420/1b46459f/attachment-0001.htm