Hi Marcos, Thanks for your comments - they are helpful. And yes, I do have a band gap in my calculations for ZnO.
I made some other observations, which are a bit wired and wanted to check if you have some thoughts on the same: 1) I am modeling nanowires with periodic boundary conditions along the length - which leads to modeling infinitely long wires. If I double the length of the nanowire unit cell along the direction in which polarization is being calculated, then should I expect to see a doubled value for polarization as well? In both the cases, essentially an inifinte wire is being modeled.. However, in one case the actual volume (and no. of atoms) modeled is doubled. That is why, I think that the value of polarization should be doubled (keeping the polarization per unit volume same). 2) On straining a unit cell, sometimes I notice weird jumps and drops in the polarization values. The slope of polarization vs strain seems to be the same accept at the jumps leading to discontinuity. Any ideas, what might be happening? Thanks in advance. Best Regards, Ravi 2009/11/30 Marcos Veríssimo Alves <[email protected]> > Ravi, > > As far as I know, anything with a band gap is an insulator... The term > "semiconductor" is generally employed only to discriminate insulators with a > band gap below a certain value, say, 4 eV. So your ZnO wires are also > insulators - provided, of course, they have a non-zero band gap from your > calculations :) > > Cheers, > > Marcos > > > On Mon, Nov 30, 2009 at 4:14 PM, Ravi Agrawal < > [email protected]> wrote: > >> Hi Chris/Andrei, >> >> Your ongoing discussion is interesting and quite insightful for me, as I >> am also planning to pursue some polarization calculations. >> >> My system will be zinc oxide nanowires - with free surfaces and >> periodicity induced along the length (modeling infinitely long wires). >> I am wondering if the approach of calculating polarization using the >> Berry-Phase approach (by employing Polarization Grids) is suitable for this >> scenario. ZnO has a band gap and is semiconducting, but not an "insulator". >> This question came up as the SIESTA manual suggests that Berry-phase >> approach is suitable only for insulators. >> >> Please, shed some light on this issue as well, in lieu of your ongoing >> discussion. >> >> Thanks, >> Ravi >> >> Northwestern University >> Evanston, IL 60208 - 3111 >> >> >> >> On Mon, Nov 30, 2009 at 9:03 AM, <[email protected]> wrote: >> >>> Dear Chris, >>> I think you attempt to cheat the code and to overcome its >>> naive protection against an attempt to calculate polarization >>> on a metal system. What matters 12 or 13 silver atoms, >>> a little bit metal is a metal, in the context given. >>> Are you sure you are doing >>> something reasonable even when "it performs the calculation!" ? >>> Do you have a band gap in your silver-on-whatever system? >>> The band gap is NEEDED for calculating the polarization, Berry phase or >>> not. >>> >>> Best regards >>> >>> Andrei Postnikov >>> >>> > 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 >>> >>> >> > -- PhD Candidate Espinosa Research Group Mechanical Engineering Northwestern University 2145 Sheridan Road, Tech Bldg. Evanston, IL 60208 - 3111 Phone: (847) - 467 - 7673
