U Asymmetric unit definition

NOTE:

It is not usually necessary to give a U card.

ITEMS READ:

U and 3 indices $h,k,l$

ASSUMPTIONS MADE:

If no U card is provided, the typical reflection indices are taken to be 13,11,10. These have the property of being all positive, with $h > k > l$.

DETAILS:

The indices $h,k,l$ are used by routine SYMUNI to select which of the possible asymmetric units it will choose. SYMUNI finds one unit, then transforms it using the space group symmetry until it contains the given $h,k,l$.

This means that $h,k,l$ must be properly inside a unit, not on a face or edge. Routine TRYUNI will give an error if this is not so.

Note that, although in general the provision of a U card will force the asymmetric unit which you intended, in some cases, notably for tetragonal groups, there are (at least) two ways of chopping reciprocal space into asymmetric units, and the user will not be able to impose one if the program has chosen the other.

EXAMPLE:

U -1,4,5

ROUTINES WHICH READ THE CARDS:

INPUTU reads one U card, and is called by SYMUNI