  A crystallographic cell is a representation of a lattice, but each lattice
can be represented just as well by any of an infinite number of such unit
cells. Searching for matches to an experimentally determined crystallographic
unit cell in a large collection of previously determined unit cells is a useful
verification step in synchrotron data collection and can be a screen for
"similar" structures, but it is more useful to search for a match to the
lattice represented by the experimentally determined cell. For identification
of substances with small cells, a unit cell match may be sufficient for unique
identification. Due to experimental error and multiple choices of cells and
differing choices of lattice centering representing the same lattice, simple
searches based on raw cell edges and angles can miss similarities among
lattices. A database of lattices using the G6 representation of the
Niggli-reduced cell as the search key provides a more robust and complete
search. Searching is implemented by finding the distance from the probe cell to
related cells using a topological embedding of the Niggli reduction in G6, so
that all cells representing similar lattices will be found. Comparison of
results with those from older cell-based search algorithms suggests significant
value in the new approach.
