In QuantumATK, most functions that are available in scripting can also be placed into a workflow using a Custom block. In a Custom block one provides a script that is executed when the workflow is run. It is also possible to specify what variables are passed into the block and which are passed out by selecting the input and output of the block.
In the case of pair defects, to put the defects into the existing MTP workflow one needs to replace the standard Defects block with one that creates pair defects. This block takes as input a bulk configuration, and outputs a Defects object. This is the same kind of object that is taken by the GenerateDefectsConfigurations block. I have attached an example workflow (in the zip file) that contains the required custom block. This script takes the input configuration, creates a VacancyGenerator, and then uses the VacancyGenerator to create a DefectPairGenerator that contains two vacancies. Different kinds of pair vacancies can be created by using either a SubstitutionalGenerator or InterstitialGenerator as the input defect generators.
Note that the general idea of pair defects is that the two defects are created within some shell around each other. The method also only produces the symmetrically unique defects in the given material. In the example workflow a 2x2x2 supercell of conventional silicon is also used. For defect pairs at larger separations, it is sometimes necessary to use larger cells to create enough distance to place the defects. In this example using the 2x2x2 supercell allows creating a second vacancy in the first, second and third coordination shells around the first vacancy. In the conventional unit cell there is only enough atoms to create the second vacancy at the first and second coordination shells.
