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Re: Permitting new physical units?

  • To: Multiple recipients of list <coredmg@iucr.org>
  • Subject: Re: Permitting new physical units?
  • From: "David Brown" <idbrown@mcmail.cis.mcmaster.ca>
  • Date: Mon, 16 Mar 1998 22:13:00 GMT
We seem to be confusing 'units' with 'dimensions'.  The discussion is not
really about units, but about the dimensions of the density that is
calculated.  Units are simple, because we have decided that the units in
the cif will always be the same (e.g. A).  If someone wants the results in
pm they can do this with their own software.  When these quantities are
give in the cif they are A.  This will be true even when the whole world is
converted to pm, because then the software will all be written to convert
the numbers found in the cif into pm.  So units are not the problem.

	In the items being discussed it is the dimension (electron density,
nuclear scattering density etc.)  These can, of course be recognised by the
units in which they are given, but once the dimension has been established,
the units are defined.  The units of electron density in a cif are always
and can only be electrons. A^-3^.  Units of C/A^3^ are not permitted.  So
we must not leave the door open to such possibilities.  The volume of the
unit cell can only be in A^3^ in a cif, so there is not question of someone
deciding to give the cell volume in m^3^ or pm^3^.

	Thus the new DDL item should not be _units_definition but
_dimensions_definition, and the allowed dimensions should be clearly
specified in the enumeration list for terms used in the regex expression.

		David Brown

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