[Pw_forum] force constants units

[Bartek Wiendlocha]

Dear Paolo,

Thank You for help with *.fc file. I've got two more questions:

1. are the force constants in Ry/bohr^2 ?
2. I want to calculate 'total' force constant matrix (3n x 3n, n= # of atoms in unit cell). So, for atom 'na', force polarization 'i' and displacement polarization 'j', I have to sum all the force_constant(m1,m2,m3,i,j,na,nb) over 'm1,m2,m3' and 'nb'. And the result will be k_j^i :
F^i(na) = - k_j^i*u_j(na)
which will give me total force on 'na' in the direction i 'F^i(na)' when moving that atom ('na') in the direction 'j' with displacement 'u_j(na)'. Am I right?

Thanks in advance,
Bartek

> 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
> So: force on atom na in direction i, when moving atom nb in direction j,
> in a cell that is R far away

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