mirror of https://github.com/abinit/abinit.git
68 lines
2.8 KiB
Markdown
68 lines
2.8 KiB
Markdown
---
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description: How to calculate transition paths
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authors: GG
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---
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<!--- This is the source file for this topics. Can be edited. -->
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This page gives hints on how to calculate transition paths with the ABINIT package.
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## Introduction
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Similarly to the PIMD, finding minimum energy paths and in particular
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transition states for chemical transformations is of great importance in many
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different fields. In ABINIT we have implemented two different flavours based
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on interpolation methods [[cite:Weinan2007]], [[cite:Henkelman2000]],
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[[cite:Mills1994]] and controlled by the keyword [[imgmov]].
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The calculation starts with the knowledge of the initial and final state
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(local minima in the configuration space) and an educated guess for the
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reaction pathway.
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If the reaction path is not given, a linear interpolation between the
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reactants and final products is constructed by a series of images
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(configurations) that connect the two states, which are given by the keyword
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[[nimage]].
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The energy path that joins the series of images is then modified at each step
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to allow the search over the lowest energy path joining the reactants and
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products.
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In the Nudged Elastic Band method (NEB), the images are connected through
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springs, with a spring constant that has to be chosen such that the images are
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uniformly spaced during the path search. The forces on each image come from
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the potential energy surface of that configuration and a spring force from the
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two closest configurations. The change in images is calculated by projecting
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out the true force perpendicular to the path and the parallel projection of
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the spring force with respect to the path [[cite:Henkelman2000]].
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The spring constant is obtained from the keyword [[neb_spring]] and the number
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of iterations is given by [[ntimimage]].
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In the String method, the system set
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up is exactly the same as in the NEB method with the difference that no spring
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constant needs to be defined. In this case, the forces are obtained as in the
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NEB method from the true force perpendicular but now the configurations are
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equally redistributed along the path at each iteration [[cite:Weinan2007]]. In
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both methods, the search stops if the number of predefined iterations
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([[ntimimage]]) or the tolerance convergence criteria ([[tolimg]]) is reached.
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As in the PIMD, each of the images can be treated in parallel and the
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requested parallelization mode is set with the keyword [[npimage]].
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Specified lattice parameters, or angles, or atomic positions, can be kept
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fixed if needed, see [[topic:GeoConstraints]].
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## Related Input Variables
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{{ related_variables }}
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## Selected Input Files
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{{ selected_input_files }}
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## Tutorials
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* [[tutorial:paral_images|Parallelism based on "images"]], e.g. for the determination of transitions paths (string method), that can be activated on top of the "KGB" parallelism for force calculations.
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