A Atomic positions etc.

ITEMS READ:

A , atom-label , x, y, z, isotropic temperature factor, scattering-factor-label, site occupation factor. The last two numbers are optional.
or
A SD ,atom-label , followed by a parallel list of standard deviations (ESD's) in the parameters on the corresponding simple A card.

ASSUMPTIONS MADE:

The scattering-factor used will be that labelled with the initial letters (terminated by the first non-letter) of the atom-label , unless a specific label is given after the isotropic temperature factor.

The site occupation factor is $=1$ unless a non-zero number is read after the (possible) scattering-factor-label.

DETAILS:

The atom-label is 1,2,3 or 4 characters, the first of which is a letter.

The scattering-factor-label is not needed at all if the scattering-factor for this atom is labelled (on an F card) with the first letters of the atom-label. (E.g. atoms Ca31 and Ca32 would both have scattering-factor Ca; atom P would have factor P, and so would atom P5, but atom Pb4 would expect factor Pb).

If the user wants the scattering-factor-label to be something other than the first letters of the atom-label, he gives the scattering-factor-label explicitly; this again has up to 4 characters, starting with a letter.

The x,y,z coordinates are fractions of a unit cell. If any of x,y,z or the isotropic temperature factor is absent, it is assumed to be 0.

ESDs need not be given for coordinates which are fixed, or related to a previous one, by symmetry. If given they will be ignored. If any are ommitted there must be a comma separated spaces in place of them.

The site occupation factor may be absent, in which case it is assumed to be 1. If the site occupation factor is wanted but the scattering-factor-label is not, the scattering-factor-label may be omitted.

If an A SD line is given without other data, then it will be filled in in the new CDF after a least squares refinement of the atomic coordinates has been made.

NOTE:

Note also that site multiplicity is taken care of by the system, and need not be artificially put into the site occupation factor by the user.

EXAMPLES:

A Ca2 .1234 .2334 2/3 1.9

There is no site occupation factor on the card, and this atom would expect a scattering-factor to be given on an F card with label Ca.

A Fe 0 1/2 1/2 0.6 Fe2 0.8
Here the atom-label is Fe, its scattering-factor is labelled Fe2, its position coordinates are (0,$1\over2$,$1\over2$), its isotropic temperature factor is 0.6 and its site occupation factor is 0.8 .

A Cu    0.1234  0.3456 1/4 0.5 
A SD Cu 0.0002  0.0003

The z parameter of Cu is fixed by symmetry so it has no ESD

A O     0.2222 0.2222 0.2476 0.750 0.980
A SD O  0.0002, ,0.0003 0.05 0.005

The y parameter of O is equal to the x parameter so its ESD is ommitted.
ROUTINES WHICH READ THE CARDS:

Routine ATOPOS calls INPUTA to read each A card and interprets the data furnished. (Routine SETFC also calls ATOPOS).