# TransPath

This page gives hints on how to calculate transition paths with the ABINIT package.

## Introduction¶

Similarly to the PIMD, finding minimum energy paths and in particular transition states for chemical transformations is of great importance in many different fields. In ABINIT we have implemented two different flavours based on interpolation methods [Weinan2007], [Henkelman2000], [Mills1994] and controlled by the keyword imgmov.

The calculation starts with the knowledge of the initial and final state (local minima in the configuration space) and an educated guess for the reaction pathway.

If the reaction path is not given, a linear interpolation between the reactants and final products is constructed by a series of images (configurations) that connect the two states, which are given by the keyword nimage.

The energy path that joins the series of images is then modified at each step to allow the search over the lowest energy path joining the reactants and products.

In the Nudged Elastic Band method (NEB), the images are connected through springs, with a spring constant that has to be chosen such that the images are uniformly spaced during the path search. The forces on each image come from the potential energy surface of that configuration and a spring force from the two closest configurations. The change in images is calculated by projecting out the true force perpendicular to the path and the parallel projection of the spring force with respect to the path [Henkelman2000].

The spring constant is obtained from the keyword neb_spring and the number of iterations is given by ntimimage. In the String method, the system set up is exactly the same as in the NEBM with the difference that no spring constant needs to be defined. In this case, the forces are obtained as in the NEB method from the true force perpendicular but now the configurations are equally redistributed along the path at each iteration [Weinan2007]. In both methods, the search stops if the number of predefined iterations (ntimimage) or the tolerance convergence criteria (tolimg) is reached.

As in the PIMD, each of the images can be treated in parallel and the requested parallelization mode is set with the keyword npimage.

Specified lattice parameters, or angles, or atomic positions, can be kept fixed if needed, see topic_GeoConstraints.

## Related Input Variables¶

*compulsory:*

*basic:*

- mep_solver Minimal Energy Path ordinary differential equation SOLVER
- tolimg TOLerance on the mean total energy for IMaGes

*useful:*

- mep_mxstep Minimal Energy Path search: MaXimum allowed STEP size
- neb_algo Nudged Elastic Band ALGOrithm
- neb_spring Nudged Elastic Band: SPRING constant
- npimage Number of Processors at the IMAGE level
- string_algo STRING method ALGOrithm

*expert:*

- cineb_start Climbing-Image Nudged Elastic Band: STARTing iteration
- dynimage DYNamics of the IMAGE
- fxcartfactor Forces to (X) CARTesian coordinates FACTOR

## Selected Input Files¶

*paral:*

*v6:*

## Tutorials¶

- 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.