Thanks Paolo, Do you think this convention is uniformly applied to any lattice with ibrav=0? (i.e., is there a general form of the convention for lattices with more than two atoms in the basis for eg.,)
Thanks again, -Navaneeth On Thu, Feb 19, 2015 at 3:35 AM, Paolo Giannozzi <[email protected]> wrote: > On Mon, 2015-02-09 at 15:45 -0800, Navaneetha Krishnan wrote: > > > I used ibrav=0 and my lattice vectors are [0 0.5 0.5], [0.5 0 0.5], > > [0.5 0.5 0] in alat units. So if I understand the convention given in > > the faq phonon section of the documentation, the first nearest > > neighbors should have the indices [1 1 1], [5 1 1], [1 5 1] and [1 1 > > 5]. > > The first nearest neighbors of atom 1 = (0,0,0) in cell at (0,0,0) are: > 1.) atom 2 = (0.25, 0.25, 0.25) in cell at ( 0.0, 0.0, 0.0); > 2.) atom 2 = (0.25, 0.25, 0.25) in cell at ( 0.0,-0.5,-0.5); > 3.) atom 2 = (0.25, 0.25, 0.25) in cell at (-0.5, 0.0,-0.5); > 4.) atom 2 = (0.25, 0.25, 0.25) in cell at (-0.5,-0.5, 0.0); > > You assume that the indices <=> lattice vector correspondence is: > [1 1 1] == (0, 0, 0), i.e. the two atoms are in the same cell > [2 1 1] == (0, 0.5, 0.5) > [5 1 1] == (0,-0.5,-0.5) (i.e. as above, refolded) > etc, yielding atoms 1 2 + [1 1 1], [5 1 1], [1 5 1], [1 1 5] as > neighbors. This makes sense (I would have made the same assumption), > but apparently the code uses a different sign convention and thinks > that the neighbors are either: > atoms 1 2 + [1 1 1], [2 1 1], [1 2 1], [1 1 2] > or > atoms 2 1 + [1 1 1], [5 1 1], [1 5 1], [1 1 5] > You may verify that these force constants have a more symmetric look > (I did). > > I am not sure where such sign convention comes from, but I am quite > sure that the interatomic force constants are used in a way that is > consistent with the convention adopted for writing them and that > results are correct. > > Paolo > > > From my understanding, the first nearest neighbours for Si have the > > diagonal term in the force constant matrix +/- alpha and off-diagonal > > term in the force constant matrix +/- beta for some alpha and beta. > > > > > > Now, the [1 1 1] force constant seems to have the right symmetries : > > atom pair : 1 2 > > 148 1 1 1 -6.40122756000E-02 11 > > 652 1 1 1 -4.29620484800E-02 12 > > 1156 1 1 1 -4.29620484800E-02 13 > > 1660 1 1 1 -4.29620484800E-02 21 > > 2164 1 1 1 -6.40122756000E-02 22 > > 2668 1 1 1 -4.29620484800E-02 23 > > 3172 1 1 1 -4.29620484800E-02 31 > > 3676 1 1 1 -4.29620484800E-02 32 > > 4180 1 1 1 -6.40122756000E-02 33 > > > > > > However, the [5 1 1] (and [1 5 1], [1 1 5]) force constants do not > > have the right symmetries > > atom pair : 1 2 > > 152 5 1 1 -3.87971851534E-03 11 > > 656 5 1 1 1.05471196450E-03 12 > > 1160 5 1 1 1.05471196450E-03 13 > > 1664 5 1 1 1.05471196450E-03 21 > > 2168 5 1 1 -3.60176689065E-04 22 > > 2672 5 1 1 -3.96403112877E-04 23 > > 3176 5 1 1 1.05471196450E-03 31 > > 3680 5 1 1 -3.96403112877E-04 32 > > 4184 5 1 1 -3.60176689065E-04 33 > > > > > > The first column is the line number in my file, then followed by the > > three indices for the lattice vectors, then the force constant and > > then finally the component (11=xx, 12=xy and so on) > > > > > > > > Does anyone have an idea why this happens? > > > > > > P.S. : In the si.scf.in file, I have CELL_PARAMETERS {cubic}. I have > > also tried with CELL_PARAMETERS {alat} and there is no difference. > > > > > > Thanks, > > Navaneeth > > -- > > Navaneetha Krishnan Ravichandran, > > Graduate Student, > > Mechanical Engineering, > > Caltech. > > _______________________________________________ > > Pw_forum mailing list > > [email protected] > > http://pwscf.org/mailman/listinfo/pw_forum > > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum > -- Navaneetha Krishnan Ravichandran, Graduate Student, Mechanical Engineering, Caltech.
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