USPEX 10.6 manual

5.7 Surfaces

To predict surface reconstructions, you have to:

  • Specify 200 or 201 : calculationType.

  • Provide a file containing substrate in VASP5 POSCAR format, as shown on Fig. 12. This can be done using online utility: https://uspex-team.org/online_utilities/substrate/ (after you get your slab model, you need to put it in your folder and run USPEX one time — it will generate for you the "final" POSCAR_SUBSTRATE file at stop. With this file you can now run your USPEX calculations).

  • Do not forget to specify vacuumSize which should be sufficently large to prevent interaction between periodic images of slabs.

  • Specify the following parameters:

\includegraphics[scale=0.5]{pic/Surface.png}
Figure 12 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
2-17
% endSymmetries

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

\(\triangleright \) variable thicknessS

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

Default: 2.0 \(\text{\r{A}}\)

Format:

3.5 : thicknessS

\(\triangleright \) variable thicknessB

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

Default: 3.0 \(\text{\r{A}}\)

Format:

3.0 : thicknessB

\(\triangleright \) variable reconstruct

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

Default: 1

Format:

1 : reconstruct

USPEX is capable of predicting surface reconstructions with variable number of surface atoms. In this case, stable surface reconstructions are dictated by chemical potentials 28 . 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
2-17
% 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:

  • Reconstructions of elemental surfaces (such as C@diamond(100) surface).

  • Reconstructions of surfaces at binary compounds (such as GaN(0001) surface).

  • Reconstructions involving foreign species on elemental surfaces (such as PdO@Pd(100) surface).