[Pw_forum] ph.x and Nesting Fermi Surfaces

[Miguel Martínez]

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
----------------------------------------

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