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Re: Removing the '_array_element_size' category or not ?

	Andy's proposal raises the distinction between 'dimensions' and
'units'.  The core dictionary maintenance group (coreDMG) is in the middle
of a similar discussion and is getting itself confused, as this
distinction is often overlooked because we frequently indicate the
dimensions by the units we use.  For example, if we specify the units as
mm, then the dimension must be 'distance', but if the dimension is
'distance' there are several possible units, e.g., m, pm, mm, A, etc. 
Usually the dimensions of a quantitity, say the cell dimensions, are
obvious, so the question is only one of deciding on the units to use (A,
nm, pm), but the coreDMG has run into the problem that the dimensions (and
hence also the units) of F(000) and the scattering density depend on the
type of radiation being used in the experiment, and this needs to be known
if the computer is to make any sense of the values.  In this case it is
impossible to define the units unless the dimensions are somehow
specified. 

	A similar problem arises for the generic arrays being proposed in
imgcif.  However, comcifs has adopted the philosophy that once the
dimensions are known, the units are uniquely defined by the dictionary and
cannot be redefined within a cif.  Thus all cifs will give the cell
constants in A.  This does not mean that the user needs to think in A
since the software can be written to convert A to nm or pm as desired. 
Cif is a transfer medium and is not written for the convenience of the
individual using the cif.  The software provides this convenience and the
software is easier to write if the units of a particular item in the cif
are always the same regardless of which program wrote the cif, or which
program reads the cif.  Current discussions in Comcifs are clearly
strongly against the idea that units should be defined within the cif. 

	However, the idea of a generic array does raise some questions,
since, if the array is macroscopic, giving the values in A might be
counterproductive, and similarly if it were microscopic, m might seem a
little unnecessary.  However, Comcifs is likely to react unfavourably to a
proposal that leaves the door wide open for user-definition of units. 
Clearly dimensions is something different, but it should be sufficient to
have a rule that, say, 'time' is always given in seconds, and that hours,
days, years etc are not an option.

	It is important to keep the concepts of 'dimension' and 'units'
separate, not only in our thinking, but also in our definitions.

			David


*****************************************************
Dr.I.David Brown,  Professor Emeritus
Brockhouse Institute for Materials Research, 
McMaster University, Hamilton, Ontario, Canada
Tel: 1-(905)-525-9140 ext 24710
Fax: 1-(905)-521-2773
idbrown@mcmaster.ca
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