[Pw_forum] force constants file

[Paolo Giannozzi]

On Monday 11 April 2005 11:23, Bartek Wiendlocha wrote:

> Could Anyone help me and write a (detailed..) description of the 
> force constants file

A description, maybe. A detailed one (or even an accurate one),
unlikely.

>   2    3  4  5.6370000  0.0000000  1.0870000  0.0000000  0.0000000
> 0.0000000 --> lattice

number of types, number of atoms in the cell, type of lattice,
lattice parameters

>            1  'Al '  -> atoms  24588.6885119930 -> ??

type #, label, atomic mass in a.u.

>            2  'B  '  -> atoms  9851.88001536857 -> ??

>     1    1      0.0000000      0.0000000      0.0000000 -> positions
>     2    2      0.5000000      0.2886751      0.5435000
>     3    2      0.0000000      0.5773503      0.5435000

atom #, type #, positions in the cell in a0 units

>  F

T if the file contains epsilon and Z*, F otherwise

>    8   8   8  -> grid size

n1 n2 n3, defining the grid of q-points in the Brillouin Zone and 
the side of the R-space supercell used to calculate the Fourier 
transform. The latter is n1*tau1, n2*tau2, n3*tau3, where 
tau1, tau2, tau3 are the primitive translations of the lattice

>    1   1   1   1  --> ??

indices: i,j,na,nb (polarization1 , polarization 2, atom 1 , atom 2)

>    1   1   1  --> ?? 1.56778730300E-01 --> force constant, but for which
> atoms?

m1 m2 m3  force_constant(m1,m2,m3,i,j,na,nb) 
where m1=1,...,n1, m2=1,...,n2, m3=1,...,n3, define a lattice vector R:
   R = (m1-1)*tau1 + (m2-1)*tau2 + (m3-1)*tau3
(if m1-1 > n1/2 and so on, refold m1-1 => m1-1-n1 and so on)
So: force on atom na in direction i, when moving atom nb in direction j,
in a cell that is R far away

> are the force constants divided by mass ?

no

> How to find the distance in real space (or the number of nearest 
> neighbours) up to which the FC are calculated?

see above: approx. n1/2, n2/2, n3/2 unit cells in the three directions

Paolo

-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy