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Re: F(000)

  • To: Multiple recipients of list <coredmg@iucr.org>
  • Subject: Re: F(000)
  • From: syd@crystal.uwa.edu.au (Sydney R Hall)
  • Date: Thu, 19 Feb 1998 14:15:16 GMT
H> I recommend coredmg to remove _exptl_crystal_F_000 from the
H> dictionary as it is useless, ill-defined and causes more bother than it
H> is worth.
H> 
H> H.

Once again our H. proves to be not_the_shrinking_violet of data definers!
A little history on this item is useful, if only to prove that tinkering 
with definitions may not always have fruitful. The first definition of 
F(000) was...

data_exptl_crystal_F_000
    _name                      '_exptl_crystal_F_000'
    _category                    exptl_crystal
    _type                        numb
    _list                        both
    _list_reference            '_exptl_crystal_id'
    _enumeration_range           0.0:
    _definition
;              The effective number of electrons in the crystal unit cell
               contributing to F(000).
;

Subsequently there was a complaint that this implied that F(000) was an
integer (which quite frankly I had always believed it to be for the Fourier
sum at least) and that some packages added the dispersion terms. In the 
spirit of "upwards compatibility" a line was added to the definition for
this eventuality. i.e.

;              The effective number of electrons in the crystal unit cell
               contributing to F(000). It may contain dispersion contributions.
;

And then there were the '96 changes that I do not think that I were involved
in the definition of (BM will correct me if my memory has failed me again)
but I have to stand behind because I gave that dictionary my vote!

In short, NO to H.'s suggestion... I agree with George that this item must
stay and that in various ways it is useful. But lets get the definition to 
match the number that is in common use.

Cheers, Syd.

PS: Howard, I haven't forgotten _refine_diff_density_min _max but I will 
need to absorb Brian's comments and get some uninterrupted time to do it.
Of course you could have first try and I could criticise... thats always
easier! :-)

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