Ian,
nearBragg2D does not contain a random number generator. I made the atom
constellations for my DS calculations using an awk script. I would
appreciate it if you could create a constellation of atoms that has the
"correlated displacements" you are talking about, so that I (and others)
can test your hypothesis with my program.
Thanks,
-James
Ian Tickle wrote:
"important". The
answer to the last question appears to be "never". The
"background"-subtracted spot intensities really are the square of the
Fourier transform of the AVERAGE electron density in a unit
cell. Yes,
the arithmetic average. Does not matter where the background comes
from. This is another beer I owe Colin.
James, without knowing more about the innards of your random-number
generator, it's not clear to me how the random number sequence that you
generate simulates the correlated displacements that are responsible for
DS, i.e. either between atoms related by lattice translations in
adjacent unit cells (-> 'acoustic' DS), or between atoms in pseudo-rigid
groups within the same unit cell (-> 'optic' DS), or even correlations
between the displacements of electrons on the same atom (-> 'Einstein'
DS). Surely the whole purpose of a 'normal' random number generator is
to avoid correlations in the generated sequence. Of course, in practice
for a sufficiently long sequence of software-generated random numbers
it's impossible to avoid correlations, but even then it's extremely
unlikely any correlations that are accidentally produced by defects in
the algorithm will simulate the physical situation. You would surely
have to deliberately design in the correlations to produce meaningful
results.
Unless you are properly simulating the set of correlated displacements
that give rise to DS, it's hardly surprising that you don't see any DS
effects in the measured intensities. I think we can take it that DS
effects are real!
Cheers
-- Ian
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