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{ BILD } { BUILD } [DELETE] iupac iupac iupac iupac real real real real real

The `BILD` command is used to specify the rules for constructing
atoms. The first four operands are atom names, and they specify a set of
four atoms which are linked together, which are referred to as I, J, K, and L.
If the third atom is prefixed by an
asterisk, then the rule is an improper torsion, where atom K is in the middle,
and atoms I, J, and L are bound to it. Otherwise, the rule is a proper torsion,
and the four atoms are bound together in a chain.

In the case of a proper torsion, the five real numbers specify the bond length between I and J, the bond angle between I, J and K, the torsion angle between I, J, K, and L, the bond angle between J, K, and L, and the bond length between K and L. In the case of an improper torsion, the five real numbers specify the bond length between I and K, the bond angle between I, K, and J, the improper torsion between I, J, K, and L, and bond angle between J, K and L, and the bond length between K and L. Units for bonds are in Angstroms; units for angles are in degrees.

In either case, the rules can be used to construct the positions of atoms I or L depending the positions of the remaining three atoms. If either bond length or either bond angle is specified as 0.0, then the program will search the parameter files for the values to use. The torsion angles are used as specified, unless they are edited by an internal coordinate editing command, see Internal Coordinates.

The `DELETE` option may be used to remove a `BILD` rule
either automatically generated, or copied using a `COPY` command.