Hi Fen,
F[+] = F_0 + Q (+x) + ...
F[-] = F_0 + Q (-x) + ...
Thus
{ F[+]/(+x) + F[-]/(-x) } / 2 = Q + ...
Regards,
Roberto
On Mon, 8 Mar 2010, Fen Hong wrote:
Dear all,
There is a subject "Problem in understanding .FC file" in Siesta-L. When
somebody explained the meaning of the *.FC file, he said "
the .FC file contains not just the elements of dynamical matrix,
but raw values of Force/Displacement, independently for each
displacement which are all done in positive and negative direction.
That's why there are 2* more elements than you expect.
In the ideal case the corresponding lines must be identical,
but in reality they are not, and the averaging over +/- helps
to maintain accuracy."
So his meaning is that we should get the average over+/- to maintain
accuracy because in positive and negative direction similar force
constant will be got . But I think not the average but the
difference/2.
siesta calculate the force constant at -x and the +x, so finally
the force constant should be
???{F1(+x)/(-d)}-{F1(-X)/(-d)}???/2
I know my thoughts maybe wrong, But I still can not undersatand it
well why is the average?
Thank you very much
Fen
