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CONStraint HARMonic FORCE real [PRINT] [REF coor-spec del] [MASS] atom-selection

`coor-spec` is a specification for the coordinates. See Read Syntax, for the syntax.

Syntactic ordering: `HARMonic` must follow `CONStraint`, and
`FORCE` must follow `HARMonic`.

The potential energy has a harmonic constraint term which allows one to prevent large motions of individual atoms. The form for this potential is as follows for coordinates:

EC = sum over all atoms of k(i)/2 * (x(i)-refx(i))**2

where *refx* is a reference set of coordinates. If `MASS` is
specified in the command line, then *k* is multiplied by the mass of
the atom resulting in a natural frequency of oscillation for the
constraint of sqrt(*k*) in AKMA units. An atom constrained with
`MASS` `FORCE` `1.0` will oscillate at 7.6 cycles/picosecond
if free of other interactions.

CONGEN supports a number of operations on the coordinate
constraints. The constraint for any atom can be set to any positive
value (specified by the `FORCE` keyword followed by the desired
value). The reference coordinates can be the current set at the point
when constraints are specified (the default) or a set can be read from a
coordinate file (specified by `REF` and a `coor-spec`). The force
constants and reference coordinates can read or written as a unit. The
`PRINT` option prints a list of all the current harmonic constraints
that are applied to the system after this command has been
executed.

It is important to understand some aspects of how the
constraints are set in order to get the most flexibility out of this
command. When CONGEN is loaded, each atom has associated with it a
harmonic force constant initially set to zero. Each call to the
`CONSTRAINT HARMONIC` command changes the value of this constant for
only those atoms specified.

The harmonic constraints may be read and written to files. The
file name to be specified in the `READ` and `WRITE` command is
CONS. The files may be read or written only in binary. The `PRINT`
command will also work for constraints. See I/O, for more details. In
addition, one may look at the contributions to the energy in detail
using the analysis facility, see Analysis.

`PRINT` specifies that a listing of of all the atoms
currently constrained should be printed out. This is done by segments
of constrained atoms, which is concise in most cases. Unfortunately in
the case of IUPAC specified constraints it is quite verbose.