[Pw_forum] correction to my previous message ab. force constants

[Bartek Wiendlocha]

I have read my previous message and I have a correction (forgive me..)
so:
2.I want to calculate 'total' force constant matrix (3n x 3n, n= # of
atoms in unit cell). So, for atom 'nb', 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'.It will give me a FC(nb,na) for atom 'nb' coming from interactions with atom 'na' (because of the 3 Newton's law, force on atom na = -force on atom nb).Then if I sum FC(nb,na) over 'na' I'll get the total force constant for atom 'nb': k_j^i(nb) :
F^i(nb) = - k_j^i(nb)*u_j(nb)
which will give me total force (from interactions with all atoms in crystal) acting on 'nb' in the direction i 'F^i(nb)' when
moving atom 'nb' in the direction 'j' with displacement 'u_j. 
Now it seems more sensible for me, but i'm not sure. Please tell me if it's OK..

Thanks again,
Bartek

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