This is an archive copy of the IUCr web site dating from 2008. For current content please visit https://www.iucr.org.
[IUCr Home Page] [CIF Home Page] [mmCIF Home Page]

more about PDB Z value

Frances Bernstein (bernstei@fcb.pdb.bnl.gov)
Fri, 27 Oct 1995 11:16:02 -0400 

This is a follow-on message to the one I sent yesterday about
needing a token for the PDB Z value, based on information provided
by Herbert Bernstein.

_cell.formula_units_Z is defined as:

;              The number of the formula units in the unit cell as specified
               by _chemical_formula.structural, _chemical_formula.moiety or
               _chemical_formula.sum.
;

But when I look at _chemical_formula I find:

;              Data items in the CHEMICAL_FORMULA category would not, in
               general, be used in a macromolecular CIF.  See instead the
               ENTITY data items.

which seems to imply that one should not be using _cell.formula_units_Z
in mmCIF.

Herbert pointed out that if one were to revise the definition of
_cell.formula_units_Z, it could be used to hold the PDB Z value.
Apparently Phil Bourne assumes that _cell.formula_units_Z is meant to
hold the PDB Z value and he uses it for that purpose in pdb2cif.

                                Frances Bernstein