The value below which the density measured using standard
               chemical and physical methods lies. The units are megagrams
               per cubic metre (grams per cubic centimetre).
               _exptl_crystal.density_meas_gt and _exptl_crystal.density_meas_lt
               should not be used to report new experimental work, for which
               _exptl_crystal.density_meas should be used.  These items are
               intended for use in reporting information in existing databases
               and archives which would be misleading if reported under
               _exptl_crystal.density_meas.