5.2 Surfaces

To perform a prediction of surface reconstructions, you have to:

Figure 10: The surface model used in USPEX. Note that POSCAR_SUBSTRATE shall exactly represent the geometrical information of its bulk crystal without vacuum. If the input has large vacuum region, the program will automatically delete it and provide a new file called POSCAR_SUBSTRATE_NEW, and this file has to be used in the calculation (renamed as POSCAR_SUBSTRATE).
% symmetries
% endSymmetries

Note: If the symmetries tag is present, USPEX will try to generate structures using plane groups.

$\triangleright $ variable thicknessS

Meaning: Thickness of surface region. Adatoms are allowed only in this region.

Default: 2.0 $\text {\r{A}}$


3.5 : thicknessS

$\triangleright $ variable thicknessB

Meaning: Thickness of buffer region in substrate. This region is part of POSCAR_SUBSTRATE, allowed to relax.

Default: 3.0 $\text {\r{A}}$


3.0 : thicknessB

$\triangleright $ variable reconstruct

Meaning: Maximum multiplications of the surface cell, to allow for complex reconstructions.

Default: 1


1 : reconstruct

USPEX also provides the possibility of studying the stable configurations with variable number of surface atoms (to be released in USPEX 10.1). In this case, stable surface reconstructions are dictated by chemical potentials 25. A typical set of input is the following:

*      TYPE OF RUN AND SYSTEM            *
USPEX  : calculationMethod (USPEX, VCNEB, META)
201    : calculationType (dimension: 0-3; molecule: 0/1; varcomp: 0/1)

% atomType
Si O
% EndAtomType

% numSpecies
2 4
% EndNumSpecies

Here we specify the maximum number of surface atoms in a $1 \times 1$ cell.

*               SURFACES                 *
% symmetries
% endSymmetries

% StoichiometryStart
1 2
% StoichiometryEnd

This defines the stoichiometry of the bulk.

-23.7563 : E_AB (DFT energy of AmBn compound, in eV per formula unit)
-5.4254  : Mu_A (DFT energy of elemental A, in eV/atom)
-4.9300  : Mu_B (DFT energy of elemental B, in eV/atom)

3.5      : thicknessS (thickness of surface region, 2 A by default )
3.0      : thicknessB (thickness of buffer region in substrate, 3 A by default)
4        : reconstruct (maximum multiplications of cell)

At the moment, USPEX supports variable-composition calculation for the following cases of surfaces: