[Phonopy-users] Force Constants in Phonopy

[<zo...@im...>]

Dear Prof Togo,

We are new to the Phonopy mail list.

We are using Phonopy & VASP to calculate the force constants in some bcc metals and compare with atomic force-constants, obtained from Born-Karman fits to neutron inelastic data. 


For example, the 1st nearest-neighbour ( 1/2a(111)) force-constants matrix has the following symmetry:

a1 b1 b1
b1 a1 b1
b1 b1 a1


with  both a1 and b1  positive.

The 2nd nearest-neighbour matrix (1/2a(200)) has symmetry

a21   0  0
0     a22  0
0    0   a22

with a21 > 0 and a22 < 0, etc.

How can we transform the force-constants matrices written in the file FORCE_CONSTANTS, generated by Phonopy using primitive cell superlattice 

( phonopy -d --dd=dim="6 6 6" -c POSCAR_PrimCell)
  
into the standard Born-Karman force constants matrices, mentioned above?

Thanks in advance for the help


Best regards


PD Dr. Nikolay Zotov
Computational material design
University of Stuttgart
70569 Stuttgart, Germany