Thanks Andrei, I was trying to use the silver system as a simple example. What I'm truly interested in calculating is the polarization in a polymer-metal nanocomposite; and what I've found is that with a matrix of 400 atoms and a metal (i.e. silver) inclusion with an odd number of atoms (I tried with 13) then SIESTA won't do the polarization calculation: odd total number of electrons. But if the inclusion is only 12 atoms, it performs the calculation! I also tried giving the system a single positive charge but to no avail.
Chris Rowan M.Sc. Candidate University of Victoria Canada 2009/11/29 <[email protected]> > > Hi, > > > > I'm wondering why polarization calculations cannot be performed when the > > system has an odd number of electrons. I've read through the derivations > > by > > Resta and the Berry phase approach, and while I follow along in a > moderate > > sense I don't see how the requirement for an even number of electrons > > enters > > the picture. > > Dear Chris, > I think this requirement enters via the demand for a system > to be non-metal. (Speaking about bulk materials, > the polarization is a property of dielectrics...) > > > For example, the polarization of a single silver atom in > > void > > cannot be calculated, but a system of two silver atoms works fine. > > I think you can polarize any isolated atom, even a silver one, > but then the Berry phase approach won't be appropriate, because > it treats the polarization current flowing through a given cross section, > in a periodic system. The periodicity is important: it permits you > to access the polarization as a bulk property, without bothering > about charge distribution at the boundaries. This does not make much > sense if your entire system is confined in your cell, so that the "surface" > is explicitly included. If your silver atom is isolated, you won't > (or at least you shouldn't) have any current flowing from it to the > adjacent cell. In this case, you can calculate polarization > straightforwardly, via dipole moment, and will have no need of Berry phase. > The same about a dimer: even if the calculation "works fine" > I'd suggest that you check what is really calculated; in my opinion > the Berry phase approach doesn't make much sense for this system either. > > Best regards > > Andrei Postnikov > > > > > Any comments much appreciated, > > > > Chris Rowan > > M.Sc. Candidate > > University of Victoria > > Canada > > > >
