checkCIF procedure

PROC-NAME: CHEMW01 Type_1

PURPOSE: To check that the _chemical_formula_weight matches that calculated for the _chemical_formula_sum .

PROCEDURE:

SUMMATION

(1) match the element symbols in _chemical_formula_sum
with values for _atom_type_symbol [NOT USED]

(2) multiply the number of atoms in _chemical_formula_sum
by the mass for this element

CALCULATE

RW = _chemical_formula_weight / SUM of (2)
DW = | _chemical_formula_weight - SUM of (2)|

TEST

IF RW
0.90 <> 1.10 issue ALERT A (> 10% difference)
   "Alert A The ratio of given/expected molecular weight as calculated from the _chemical_formula_sum lies outside the range 0.90 <> 1.10 "
0.95 <> 1.05 issue ALERT B (> 5% difference)
   "Alert B The ratio of given/expected molecular weight as calculated from the _chemical_formula_sum lies outside the range 0.95 <> 1.05 "
0.99 <> 1.01 issue ALERT C (> 1% difference)
   "Alert C The ratio of given/expected molecular weight as calculated from the _chemical_formula_sum lies outside the range 0.99 <> 1.01 "

IF DW > 1.0 and category is FO, FM, CO or CM
issue ALERT C
   "Alert C The difference between the given and expected weight for compound is greater 1 mass unit. Check that all hydrogen atoms have been taken into account."

The formula weight given in the CIF differs from that calculated using the formula in _chemical_formula_sum. Check that (a) you have calculated the formula weight correctly and (b) that _chemical_formula_sum is correctly and fully specified. Remember that _chemical_formula_sum and _chemical_formula_weight must correspond with the true sum of all elements in all distinct moieties present, including any solvent and all H atoms, and not the contents implied by the model if there are differences due to the omission of a few H atoms or solvent atoms. If multiple moieties such as solvent are present, or one of the moieties possesses or is disordered about a crystallographic symmetry element, ensure that the correct ratio of moieties has been allowed for in the calculation of Z, _chemical_formula_sum and the formula weight. _chemical_formula_sum must also be in the correct format (e.g. do not use subscript formatting: C10 H20 is correct, C~10~ H~20~ is not).

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