CHEMW02

PROC-NAME: CHEMW02 Type_1

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

PROCEDURE:

SUMMATION

(1) match the element symbols in _chemical_formula_moiety with values for _atom_type_symbol

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

CALCULATE

RM = _chemical_formula_weight / SUM of (2)
DM = | _chemical_formula_weight - SUM of (2)|

TEST

IF RM
0.90 <> 1.10 issue ALERT A (> 10% difference)
   "Alert A The ratio of given/expected molecular weight as calculated from the _chemical_formula_moiety 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_moiety 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_moiety lies outside the range 0.99 <> 1.01"

IF DM > 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_moiety. Check that (a) you have calculated the formula weight correctly and (b) that _chemical_formula_moiety is correctly and fully specified. Remember that _chemical_formula_moiety 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_moiety and _chemical_formula_weight.

Note that _chemical_formula_moiety must also be in the correct format. Very often CHECKCIF is unable to interpret _chemical_formula_moiety because it has been specified incorrectly (e.g. a common mistake is to attempt to depict organometallic coordination). Further information about correctly specifying _chemical_formula_moiety is give "here" (link to my text given under CHEMM_01).

Full list of validation algorithms