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Given the positions of any three atoms, the position of a
fourth atom can be defined in relative terms (internal
coordinates) with three values: a distance, an angle, and
a dihedral specification. Where many atoms are connected
in a long sequence (as in proteins) it is easiest to consider
four atoms in a chain. If the positions of one end of the
chain is known, it is possible to find the positions of all
of the remaining atoms with a series of internal coordinate
values. But in the more general case, where some central
portion of a molecule is known it is necessary to be able
work in both directions. This lead to the present form of
the internal coordinate data structure (five values for four
atoms) where if either endpoint is unknown and the other three
atoms are determined, the position of the end atom can be found.
The improper type of internal coordinate data structure was
created for branching structures (as opposed to simple chains).
Since there are roughly five values in the data structure for
every atom it is clear that the positions are overspecified
Keep this in mind when externally editing IC files.
The program will use the first acceptable value when building
a structure and ignore any redundancies. The `EDIT` commands
will always modify all occurrences of each edited parameter.