Dear Paul, If I understood you correctly, you want to run a ph.x calculation at a q of your choice such that q=2kf, right? In that case, you only need to specify ldisp =.false., not specify any nq1, nq2, nq3 and *after* the slash in the ph.x input, specify the q points at which you want the calculations done.
Example 02 has ph.x calculate the phonon frequencies at X (and gamma) for Fm-3d Si and C, and fcc Ni at X. Hope it helps, Miguel On Sat, 5 Mar 2011 18:08:30 -0800 "W2AGZ" <w2agz at w2agz.com> wrote: > To All: > > > > I'm running some exercises on electron-phonon interactions in narrow > band metal monolayers (actually tetragonal unit cells with "large" c/a > parameters...and think doped transition metal oxides). In this > quasi-2D system, several of the Fermi surfaces approach planarity > (actually, "Fermi lines"), suggesting strong electron-phonon > interaction should be driven by the "nesting" wave vector (think CDW > and/or Peierls-Froehlich). > > > > However, I'm having difficulty seeing how to structure the input to > ph.x to select this wavevector alone (or indeed if this is even > possible with ph.x). Imagine a square-planar BZ with dimensions (0.5, > 0.5) in 2pi/a units, with a "Fermi line" perpendicular to, and kf > distance along the (1,1) direction from the BZ origin. Now, as far > as I can see from the ph.x documentation, nq1, etc., is only for > defining the Monkhorst-Pack grid for the desired phonon grid-mesh > (and are integers anyway). In principle, one could set the "line of > input" xq's to the desired kf (fractional values are presumably > allowed), but which would have its origin at BZ center, whereas one > would like to "drive" or "excite" the phonons at -kf, +kf together. > > > > I hope I've made the issue/question clear and haven't said anything > egregriously stupid. > > > > Thanks in advance, > > > > Paul Grant > > IBM Research Staff Member Emeritus > > Visiting Scholar, Stanford > -- ---------------------------------------- Dr. Miguel Martinez Canales Department of Physics & Astronomy University College London Gower Street, London, WC1E 6BT (UK) Fax: +44 207 679 0595 Tlf: +44 207 678 3476 ---------------------------------------- "If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas." George Bernard Shaw
