8.8. The NEB section

The $neb section includes a subsection for each nudged-elastic-band hops.

  • Each neb hop should be a subsection labeled with a name composed of a starting and ending defect group, connected with a dash, like vac1-vac2.

  • The subsection should also include the movement of each primary moving atom, including:

    • The atom’s element symbol: if an elementmap subsection is given in The Structure section, then the mapped designations X1, X2, and so on can be given instead of an element symbol.
    • The starting and ending .defect group. as specified in the $defects section, and then also indicate the movement of elements, and their closest starting and ending positions. These explicit positions disambiguate between possible interpolations.
    • The images keyword, which specifies the number of intermediate images.

Again, the $neb section is tied to specific defect labels. The NEB ingredients must be able to find defects or defect groups with those labels.

8.8.1. Charges for NEBs

To enable charged-supercell NEBs, use <Q> tags for the defect and NEB ingredients in The Recipe section and also specify charges for the defects in The Defects section.

The NEB ingredients will only be run for charges in the charge ranges of both parent endpoints.

For example, if defect parent vac1 has a charge range of charge=-2,0 and defect parent vac2 has a charge range of charge=-1,3, then the NEB with the label vac1-vac2 will only run with supercell charges -1 and 0.

8.8.2. Phonons for NEBs

Phonons may be specified within each NEB grouping, as in The Defects section.

The presumed saddle point in an NEB is usually taken.

  • To give the saddle point structure to the phonon calculation, in The Ingredients section, use mast_update_children give_saddle_structure for the NEB ingredient type of the NEB parent to the phonon calculation.
  • If the frequencies of a moving atom are desired for the phonon calculations, and if that atom is anticipated to pass from fractional coordinate 0 0 0 to fractional coordinate 0.5 0 0, then the phonon_center_site should be 0.25 0 0 (assuming a straight path), and the phonon_center_radius is probably about 1 Angstrom.

Example defect and NEB section together:

$defects

coord_type fractional
threshold 1e-4

vacancy 0.0 0.0 0.0 Mg label=vac1
vacancy 0.0 0.5 0.5 Mg label=vac2
interstitial 0.25 0.0 0.0 Al label=int1
interstitial 0.0 0.25 0.0 Al label=int2

$end

$neb

begin vac1-vac2
images 1
Mg, 0 0 0, 0 .5 0.5
end

begin int1-int2
Al, 0.25 0 0, 0 0.25 0
images 3
phonon movingatom 0.125 0.125 0.0 1.0
end

$end