[Pw_forum] Query regarding real space interatomic force constant

[Stefano Baroni]

Dear Sandeep,

sure it can, if you take all the periodic replica of any atom into account (as one should).

SB

On Jun 30, 2011, at 7:28 PM, Sandeep Kumar wrote:

> Dear PWSCF users,
> I have carried out linear response phonon calculations for Al over a 4 * 4 * 4 grid of q-points. This corresponds to a calculation in a supercell comprised of 4 * 4* 4 unit cells (primitive).
> We recall that the lattice vectors in fractional coordinate system of a fcc primitive cell are-
> a=(0.5, 0.5, 0.0)
> b=(0.0, 0.5, 0.5)
> c=(0.5, 0.0, 0.5)
> 
>> From the force-constant file (Al44.fc)  I obtained the force constant matrix Phi(i1,i2, na, nb) for pairs of atoms in the super-cell with the relative positions given  by
> R = (i-1)*a + (j-1)*b +(k-1)*c
> where i j and k are read from Al444.fc file.
> However, such a supercell does not include the pair of atoms located along the cube axis direction i.e R = (a,0,0) or (0,a,0) or (0,0,a), and hence the calculation CAN NOT give us the force-constant matrices along those pair of atoms.
> How do we obtain the force-constant matrices for pair of atoms located along the cube axes direction.
> I will be VERY VERY GRATEFUL to any help in this regard.
> 
> Sandeep K. Lahiri
> Department of Mechanical Engineering
> Massachusetts Institute of Technology
> Cambridge, MA
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
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