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Using this form of the `CONS` command, one may put
constraints on the dihedral angles formed by sets of any four atoms.
The constraints may be set to either a single angle, or to be bounded by
two angles (a flat bottom well). The improper torsion potential is used
to maintain said angles.

The command for setting the dihedral constraints is as follows:

CONStraint { DIHEdral [BYNUM] dihe-spec FORCE real MINimum real MAXimum real } { CLDH }

Syntactic ordering: `DIHEDRAL` or `CLDH` must follow
`CONSTRAINT`, and `FORCE` and `MIN` must follow
`DIHEDRAL`.

If `BYNUM` is specified, then

dihe-spec ::= integer integer integer integer

If `BYNUM` is not specified, then

dihe-spec ::= atom-spec : atom-spec : atom-spec : atom-spec :

where:

atom-spec ::= segid resid iupac

Note that colons must be used as delimiters following each
`atom-spec`.

`DIHEDRAL` adds a torsion angle to the list of constrained
angles using the specified atoms, force constant, and minimum and
maximum dihedral angles. `CLDH` clears the list of constrained
dihedrals so that different angles or new constraint parameters can be
specified.

When a range of angles is specified, it is important to keep in mind
that torsion angles are periodic. Therefore, the `MINIMUM` and
`MAXIMUM` bounds are taken literally. Reversing the order of
values specified for these variables has the effect of complementing
the range of the angular constraints. For example, a specification
of `MINIMUM 175 MAXIMUM -175` is just 10 degrees, whereas the
specification of `MINIMUM -175 MAXIMUM 175` is 350 degrees.

In order to simplify the specification of constraints, there are a number
of defaulting rules used by this command. First, for each atom which follows
the first, you can omit `segid`'s, `resid`'s, and `iupac` names
if they match values from the previous atom. Be careful here if any of
the identifiers are also used for higher order structures. For example,
if a protein has a `segid` of 1 and a `resid` of 1, then
specifying a single 1 will be interpreted as a segment identifier. In all
cases, use the `PRINT` `CONS` command to check that you got what
you expected.

Next, omitted `FORCE` values will be taken from the previous
constraint if it exists. `MINIMUM` and `MAXIMUM` values will
also default to previous values if neither option is specified.
However, if either one is specified, but the other is missing, then
they default to each other, so that the effect is to use a single point
harmonic well.

Do not use a value of -9999 as the minimum or maximum dihedral angle, since the program uses this value to indicate that no angle was found on the command line.

These constraints can be coupled to a conformational search. See the
option, `EPCONS`, in Miscellaneous Global Options.

The `PRINT` `CONS` command, see Print Command,
will work for constraints. As of now (October 13, 1992), one cannot
analyze the contributions of this term using the analysis facility nor
can one read or write the description of this term out. Someday
...