CHIV Instruction
CHIV V [0] s [0.2] atomnames
The chiral volumes of the named atoms are restrained to the value V (in cubic
Angstroms) with standard deviation s. The chiral volume is defined as the
volume of the tetrahedron formed by the three bonds to each named atom, which
must be bonded to three and only three non-hydrogen atoms in the connectivity
list; the order in the connectivity list, which is determined by the order of
increasing bond lengths, defines the sign of the chiral volume. Note that
RTAB may be used to list chiral volumes defined in the same way but without
restraining them. The chiral volume is positive for the alpha-carbon (CA) of
an L-amino-acid if the order of the three bond lengths is CA-N, CA-C, CA-CB
(as would be expected for an accurate structure). Note that 'CHIV 0' (or just
CHIV since the default V is zero) may be used to impose a planarity restraint
on an atom which is bonded to three others (by making the chiral volume zero),
and is mathematically equivalent to a FLAT instruction which names the four
atoms explicitly.