[Imgcif-l] ImgCIF Densiry Maps
Joe Krahn krahn at niehs.nih.govThu Mar 22 17:45:35 GMT 2007
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How many people are working on the new electron-density dictionary? It is useful to include a space-group name, which should be sufficient to define the symmetry for standard settings, as defined by the International Tables. Individual symmetry operator definitions are also valuable. Most current software defines symmetry operators with the simple equation form: -Y,Z+1/2,-X It turns out that these equations are very easy to parse, as long as they are written the 'normal' way, which means that any numeric value is a simple fraction of single-digit numbers. So, the trouble of parsing these equations can be worth avoiding the extra work of defining matrix+vector based symmetry. Since this is an established method in crystallography, it seems to me to be the best approach. However, there are certainly many non-crystallographic symmetry forms which really require a matrix+vector form. The disadvantage of a matrix form is that many digits are needed for fractions like 1/6, and even then there is a bit of round-off error. So, if the equation form is not used, I propose that there should be a way to define the translation vector in fractional units. It is possible to write all needed symmetry translations as integers in 1/24th units. So, matrix/vector symmetry should be supported. Are there opinions as to whether the equation method should or should not also be supported for things like electron-density maps? Joe Krahn
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