Hi Brian,
In fact my problem came from a default parameter in LAZY but not from
thermal term (i had checked it was at 0),it was the correction for
anomalous dispersion which was made by LAZY.
Patrick Weisbecker.
At 10:26 05/03/99 -0500, you wrote:
>Hi Pat,
>
>Not everything requires a computer, even though I have a reputation for
>liking to use them.
>
>Going back to Stout & Jenson, one finds
> F(hkl) = SUM f(i) EXP[2 pi i * (hx + ky + lz)]
>[since you are ignoring displacement (nee thermal) factors]
>
>For the FCC case, you have 4 atoms at (0 0 0), (1/2 1/2 0), (0 1/2 1/2),
>(1/2 0 1/2)
>So if you pick one of your reflections, say the 111, then F(111) = 4
>f(Ni)
>
>For lambda= 1.5418, 2theta = 44.53 (I cheated a bit here and used a
>computer to get 2theta rather than looking up a0 for Ni in a book) and
>sin(theta)/lambda = 0.246 ~= 0.25,
>so f(Ni) ~= 20.39, using Volume C.
>
>so F(111) ~= 81.56, which is pretty close to the powdercell value and
>would probably be closer if I interpolated to get f at
>sin(theta)/lambda=0.246 rather than sin(theta)/lambda=0.25
>
>Why then does lazy-pulverix give 77.4?
>
>Well, looking at the printout, one sees that the default for B is 1.0,
>so adding the "thermal" term, exp[-B * sin2(theta)/lambda2] = 0.94, I
>get
>
>F(111) ~= 76.78, which is pretty close to the lazy-pulverix value.
>
>End of mystery, I hope. I will leave computing F for the other two
>reflections as "an exercise for the reader."
>
>********************************************************************
>Brian H. Toby, Ph.D. Leader, Crystallography Team
>[EMAIL PROTECTED] NIST Center for Neutron Research, Stop 8562
>voice: 301-975-4297 National Institute of Standards & Technology
>FAX: 301-921-9847 Gaithersburg, MD 20899-8562
>********************************************************************
>
>
