Dear Paolo, Thanks for your reply. I found the information I needed in ‘phq_setup’ and other Phonon subroutines which generate ‘u’ initial displacement pattern which are also available in xml format. I should have checked them before posting my question! Thanks for your time.
Yours, Sam > On 1 Mar 2017, at 15:53, Paolo Giannozzi <[email protected]> wrote: > > Displacement patterns used by the code depend upon the symmetry (and > upon an obscure stochastic algorithm that makes them irreproducible) > only because the linear-response machinery makes usage of symmetry to > reduce the amount of computation. In principle you might use whatever > linear combination of atomic displacements spanning all vibrational > degrees of freedom (3*N for a system containing N atoms); or, simply, > atom 1 along x,y,z; atom 2 along x,y,z,; etc. > > Displacement patterns are used only to compute the linear response and > the dynamical matrix. Once you have the dynamical matrix, you can > forget about them (not that you need to know anything) and compute > phonon displacement patterns (this is what you need to know about). > Phonons are not determined uniquely by symmetry, except in special > cases. > > Paolo > > On Wed, Mar 1, 2017 at 2:54 PM, Azadi, Sam <[email protected]> wrote: >> Dear Paolo, >> >> Thanks for your answer. >> You pointed out that the “given displacement pattern” only depends on >> the symmetry of the unperturbed crystal and are chosen along irreducible >> representations >> of the symmetry group. >> Sorry if I’m slow, but I still don’t understand where does the “given >> displacements pattern” come from? >> As you said there is a given displacement pattern and then the linear >> response formalism is employed. >> Do you mean it’s only depend on the symmetry of system? >> If I perform DFPT phonon calculations at the Gamma point for H atoms in BCC >> lattice and Fe atoms >> arranged on BCC lattice, am I supposed to see the same “displacement >> pattern” ? >> I agree that the normal coordinates vector are normalised. >> >> Thanks in advance. >> >> Yours, Sam >> >> >> >>> On 1 Mar 2017, at 12:44, Paolo Giannozzi <[email protected]> wrote: >>> >>> On Wed, Mar 1, 2017 at 1:22 PM, Azadi, Sam <[email protected]> wrote: >>> >>>> 3N normal coordinates related to 3*N normal modes are displaced compared >>>> to relaxed positions, if I’m right. >>> >>> Nothing is actually "displaced": what is computed is the linear >>> response to a given displacement pattern, using perturbation theory. >>> The amplitude of the perturbation does not affect the calculation. >>> >>> Specific displacement patterns depend upon the symmetry of the >>> (unperturbed) crystal and are chosen along irreducible representations >>> of the symmetry group. I think they are normalized but I don't >>> remember exacty how. >>> >>> Paolo >>> -- >>> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche, >>> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy >>> Phone +39-0432-558216, fax +39-0432-558222 >>> >>> _______________________________________________ >>> 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 > > > > -- > Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche, > Univ. Udine, via delle Scienze 208, 33100 Udine, Italy > Phone +39-0432-558216, fax +39-0432-558222 > > _______________________________________________ > 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
