# Geometry constraints

### This page gives hints on how to constaint the geometry of the system in geometry optimization, molecular dynamics or searches with the ABINIT package.

##### Copyright (C) 2016-2017 ABINIT group (GG)

Mentioned in topic_GeoOpt, topic_MolecularDynamics, topic_TransPath, help_features#4.

### Table of content:

- 1. Introduction.
- 2. Related input variables.

### 1. Introduction.

There are two mechanisms to put constraints on the atom positions in ABINIT. They can be used in geometry optimization, molecular dynamics (including PIMD) or other geometry algorithms (e.g. transition path searches).

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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### 2. Related input variables.

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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