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<div class="moz-cite-prefix">On 10/27/14 4:43 PM, Nathaniel Echols
wrote:<br>
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cite="mid:CALeAa1MapZosx+898mrWJ8gudT9hhYgQ4jZYpWr64Z7wLwTLmQ@mail.gmail.com"
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<div dir="ltr">On Mon, Oct 27, 2014 at 1:19 PM, George
Devaniranjan <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:devaniranjan@gmail.com" target="_blank">devaniranjan@gmail.com</a>></span>
wrote:<br>
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<div>Would you define "significant" for me (as you see
it of course)?</div>
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<div>Pavel's definition:</div>
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<div><a moz-do-not-send="true"
href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2906258/">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2906258/</a><br>
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<div>"...the difference between R factors computed using the
different methods is typically less than 0.01%."</div>
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<div>I think this is probably a typo and it is supposed to
mean "1%" or "0.01", which would have been my estimate.�
Certainly differences below 0.005 are hardly worth
noticing, and below 0.001 is statistical noise.�
Differences above 0.01 are more worrisome (although not
entirely unheard of).<br>
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It depends what you compare and how.. Two scenarios:<br>
<br>
1) Compute Fcalc using FFT (Fc_fft) and direct summation (Fc_direct)
and compute R-factor(Fc_fft, Fc_direct). In this case indeed
typically the R-factor will be below 1% (not 0.01% !). Attached
script illustrates this (to run: phenix.python run.py).<br>
Also, Table 4 in<br>
<br>
Acta Cryst. (2004). A60, 19-32.<br>
On a fast calculation of structure factors at a subatomic resolution<br>
P. V. Afonine and A. Urzhumtsev<br>
<br>
does exactly this comparison.<br>
<br>
2) You run two identical refinements, in one you use FFT and in the
other one direct summation. In this case the difference between
R-factors is likely to be below 0.01%. This is because refinable
parameters will absorb the differences.<br>
<br>
Pavel<br>
<br>
<br>
<br>
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