It might happen if you transform one atom into another like in the "computational alchemy" approach [PRL 66 2116( 1991)] stefano
On 04/20/2011 09:51 PM, David Strubbe wrote: > 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 >> >> > > > _______________________________________________ > 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/20110420/15dafa98/attachment.htm