Dear Hailiang,
This apparently is not the real physics, since the
electron density has to be positive everywhere (hope I am right).
Yes, you are right when you are talking about the electron density.
You are wrong when you are talking about a Fourier synthesis calculated
always at a finite
Hi
Sacha is absolutely right here. This was made plain to me during a plenary
session at the recent BCA meeting in Warwick, given by a powder (not protein)
crystallographer - who was using histogram matching density modification, with
negative densities; an expert in density modification in
Dear Sacha:
Yes, I think Fourier synthesis at a finite resolution range will generate
some negative, or more generally imaginary values in real space (hope I am
right again:). For the imaginary values, I think the map should take the
amplitude of it (maybe I am wrong). Do they normally make the
Uhh. No. You will only get imaginary electron density if your
structure factors violate Friedel's law. I am not aware of map
calculation codes that do this (on purpose).
BTW, imaginary electrons are really just slow electrons that don't
respond to the x-rays as fast as the average electron
Dear Hailiang,As James said, the hermitian symmetry of Fourier coefficients,
F(h)=F*(-h), that is known in diffraction theory as the Friedel's law, is an
equivalent of the condition that the corresponding function (electron density)
is a real function.I think if you need further information you
Hi there:
I found that the grid values in the map file generated by CCP4-fft
generally has a mean value of ~0, and of course there will be lots of
negative values. This apparently is not the real physics, since the
electron density has to be positive everywhere (hope I am right). Can
somebody
Hailiang Zhang wrote:
Hi there:
I found that the grid values in the map file generated by CCP4-fft
generally has a mean value of ~0, and of course there will be lots of
negative values. This apparently is not the real physics, since the
electron density has to be positive everywhere (hope I am
Edward A. Berry wrote:
Hailiang Zhang wrote:
Hi there:
I found that the grid values in the map file generated by CCP4-fft
generally has a mean value of ~0, and of course there will be lots of
negative values. This apparently is not the real physics, since the
electron density has to be
