EQIV $n symmetry operationDefines symmetry operation $n for referencing symmetry equivalent atoms on any instruction which allows atom names, by appending '_$n' (where n is an integer between 1 and 511 inclusive) to the atom name. Such a symmetry operation must be defined before it is used; it does not have to be an allowed operation of the space group, but the same notation is used as on the SYMM instruction. The same $n may not appear on two separate EQIV instructions. Thus:
EQIV $2 1-x, y, 1-z CONF C1 C2 C2_$2 C1_$2could be used to calculate a torsion angle across a crystallographic twofold axis (note that this may be required because CONF with no atom names only generates torsion angles automatically which involve the unique atom list and a one atom deep shell of symmetry equivalents). If the instruction codeword refers to a residue, this is applied to the named atoms before any symmetry operation specified with '_$n'. Thus:
RTAB_23 O..O OG_12 O_$3would calculate the (hydrogen bond) distance between OG_12 and (O_23)_$3, i.e. between OG in residue 12 and the equivalent obtained by applying the symmetry operation defined by EQIV $3 to the atom O in residue 23.
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