The simplest one (entry point iatfix) simply define a set of atoms that are fixed, either entirely, or only along one of the directions (for the latter, see the warning in iatfix)
A more complex one, but also much more powerful, allows to place constraints on linear combinations of atomic positions.
Thanks to such constraint, the mean position of two atoms (or a fragment, like a molecule) can be fixed,
or constrained to stay within an arbitrary plane. One can thus also sample different mean positions.
See a complete description in wtatcon.
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Basic input variables:
... iatfix [Indices of AToms that are FIXed ]
... iatfixx [Indices of AToms that are FIXed along the X direction]
... iatfixy [Indices of AToms that are FIXed along the Y direction]
... iatfixz [Indices of AToms that are FIXed along the Z direction]
... natfix [Number of Atoms that are FIXed]
... natfixx [Number of Atoms that are FIXed along the X direction]
... natfixy [Number of Atoms that are FIXed along the Y direction]
... natfixz [Number of Atoms that are FIXed along the Z direction]
Useful input variables:
... iatcon [Indices of AToms in CONstraint equations]
... natcon [Number of AToms in CONstraint equations]
... nconeq [Number of CONstraint EQuations]
... wtatcon [WeighTs for AToms in CONstraint equations]
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