Hi Ed,<div><br></div><div><div class="gmail_quote">On Mon, Jul 11, 2011 at 3:35 PM, Edward A. Berry <span dir="ltr"><<a href="mailto:BerryE@upstate.edu">BerryE@upstate.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
It seems to me there are two things that could be meant by "expand to P1"<br>
<br>
One is when data has been reduced to the Reciprocal Space<br>
asymmetric unit (or otherwise one asymmetric unit of a<br>
symmetric dataset has been obtained) and you want to expand<br>
it to P1 by using symmetry to generate all the<br>
symmetry -equivalent reflections.<br></blockquote><div><br></div><div>This is what the <font class="Apple-style-span" face="'courier new', monospace">cctbx.miller.array.expand_to_p1()</font> method does. -- It is a very simple algorithm once you have the list of symmetry operations.</div>
<div>�</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
The other is where a full P1 dataset has been calculated from just<br>
one asymmetric unit of the crystal (and hence does not exhibit the<br>
crystallographic symmetry) and you want to generate the transform<br>
of the entire crystal. (I think this is how all the space-group -<br>
specific fft programs used to work before computers got so fast it<br>
was less bother to just do everything in P1 with the whole cell)<br>
Presumably this involves applying the real space symmetry<br>
operators to get n rotated (or phase-shifted for translational<br>
symmetry) P1 datasets and adding them vectorially.<br></blockquote><div><br></div><div>�I tend to call this "symmetry summation in reciprocal space". You are correct, this is used when we compute structure factors with the FFT method.</div>
<div><br></div><div>Ralf</div><div><br></div></div></div>