5.4 Surfaces

To predict surface reconstructions, you have to:

\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: