[Pw_forum] IFC's

Ezad Shojaee ezadshojaee at hotmail.com
Sun Sep 2 18:21:09 CEST 2007

consider one special element of the dynamical matrix, if we calculate IFC's by a 3*3*3 grid, it seems that the code generates 27 real space IFC's to be used in calculating the same element of the dynamical matrix for an arbitrary point, as the file containing IFC's for example  "###.fc"  is
1  1  3     6  ( indicating the special element)
1  1  1     C(R)
2  1  1      .....
3  3  3     C(R')
the code calculates C(q) by  sum {C(R)exp(iq.R)}, so does the code use 27 R's for the summation to calculate the element? and does the code defines the 'R' with the 3 numbers in front of the C(R)'s in ###.fc? if it is so, imagine we calculated by a 4*4*4 grid and the ###.fc file containing IFC's in the real space is
1  1  3    6  ( indicating the same element of DM)
1  1  1    C(R'')
4  4  4    C(R''')
if the 'R' used in the first file to calculate C(R) is the same used to calculate C(R'') in the second file, should we sum the C(R) and C(R''=R) to obtain more accurate figure for IFC's (27 of them) because with this, we have used more q points to calculate C(R)'s in the 27 first among 64 ? 
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