In some PDB entries that were determined by NMR, we have the following types of atoms: REMARK 7 PSEUDO-ATOMS DESIGNATED AS Q ARE DIMENSIONLESS REFERENCE REMARK 7 POINTS REPRESENTING A GROUP OF HYDROGEN ATOMS. THEY ARE REMARK 7 PLACED IN THE CENTER OF THE POSITIONS OF THE HYDROGEN ATOMS REMARK 7 THEY REPRESENT. QA REPRESENTS THE TWO METHYLENE HYDROGEN REMARK 7 ATOMS OF GLY. QB, QG, ... REPRESENT BETA, GAMMA, ... REMARK 7 METHYLENE OR METHYL GROUPS IN THE SIDE CHAINS. IN CASE OF REMARK 7 BRANCHES IN THE SIDE CHAINS THE NUMBERS OF THE PSEUDO-ATOMS REMARK 7 ARE THE SAME AS THE NUMBERS OF THE CARBONS TO WHICH THE REMARK 7 HYDROGEN ATOMS ARE ATTACHED. In the mmCIF dictionary there is the following field: save__atom_site.calc_flag _item_description.description ; A standard code to signal if the site data has been determined by diffraction data or calculated from the geometry of surrounding sites, or has been assigned dummy coordinates. The abbreviation 'c' may be used in place of 'calc'. ; _item.name '_atom_site.calc_flag' _item.category_id atom_site _item.mandatory_code no _item_aliases.alias_name '_atom_site_calc_flag' _item_default.value d _item_type.code code loop_ _item_enumeration.value _item_enumeration.detail d 'determined from diffraction measurements' calc 'calculated from molecular geometry' c 'abbreviation for "calc"' dum 'dummy site with meaningless coordinates' It appears to me that the Q-type atoms must appear as atom_site records because there is no other way to represent them. The question then arises of whether an additional flag should be included under _item_enumeration.detail to identify these Q-type atoms. Frances C. Bernstein