# Workflow for interfacial properties

Here we release a computational protocol to calculate two intrinsic tribological properties of solid interfaces from first principles, namely the adhesion energy, γ, and the ideal interfacial shear strength, τ. These properties, which correspond to the energy required to separate two surfaces from contact and to the static friction force per unit area, respectively, are ruled by physical/chemical interactions between the surfaces in contact. First principles calculations based on Density Functional Theory (DFT) can accurately describe surface-surface interactions, offering the possibility to characterize the adhesive and shear strengths of materials in silico. We implemented the computational protocol as an AiiDA workflow that allows to obtain the γ and τ figures of merits in a high throughput manner. The software we produced uses a simple input file and most computational parameters determined automatically.

The figure above offers a schematic representation of the workflow, and the most important outputs, which correspond to the green boxes, are listed below.

1. The kinetic energy cutoff for the wavefunctions is converged to the selected accuracy using a very dense k-point grid.
2. The minimal k-point density able to satisfy the selected accuracy criteria is determined. This density is then used for all subsequent calculations to ensure minimal computational cost.
3. The lattice parameter is optimized using a simple bulk cell.
4. A surface slab is constructed according to the Miller indices provided by the user and the surface energy σ is calculated.
5. We construct the potential energy surface (PES) by displacing two slabs laterally (and relaxing them in the perpendicular direction) using high symmetry points for efficiency and then interpolating between them using radial basis functions. With the full knowledge of total energy as a function of lateral displacements, we are also able to identify the most likely sliding path, which connects the minima of the PES by traversing the lowest saddle points. This is known as the minimum energy path (MEP).
6. The perpendicular potential profile, γ(z), is then obtained by displacing the two slabs normal to the interface plane. Thus we are able (at the minimum and maximum values of the lateral PES) to describe the distance dependence of the interaction, both in attractive and repulsive regimes.
7. For the same minimum and maximum positions of the PES we calculate the electronic charge displacements.
8. The adhesion energy γmin is determined as the minimum of the potential energy surface.
9. The ideal interfacial shear strength $\tau$ can be determined in various directions. In our approach it is computed for two high symmetry directions orthogonal to each other and for the minimum energy path, along which sliding occurs with the highest statistical weight.

Last software release:

https://github.com/mcrighi/interface-workflow