4.6 Cell

It is useful to create all new structures (before relaxing them) with a unit cell volume appropriate for given conditions. This can be specified in the Latticevalues keyblock:

variable Latticevalues

Meaning: Specifies the initial volume of the unit cell or known lattice parameters.

Default: For cell volumes you don’t have to specify values — USPEX has a powerful algorithm to find reasonable estimates at any pressure.

Format:

% Latticevalues
125.00
% Endvalues

Notes: (1) This volume is only used as an initial guess and only influences the first generation, each structure is fully optimized and adopts the volume corresponding to the (free) energy minimum. This keyblock also has another use: when you know the lattice parameters (e.g., from experiment), you can specify them in the Latticevalues keyblock instead of unit cell volume, e.g.:

% Latticevalues
7.49 0.0 0.0
0.0 9.71 0.0
0.0 0.0 7.07
% Endvalues

You can also specify unit cell parameters just by listing a, b, c, , , and values:

% Latticevalues
10.1 8.4 12.5 90.0 101.3 90.0
% Endvalues

Attention: if you do a calculation with a fixed monoclinic cell, please use setting with special angle (standard setting).

(2) For variable-composition calculations, you have to specify the volume of end members of the compositional search space, e.g.:

% Latticevalues
12.5 14.0 11.0
% Endvalues

(3) Users no longer need to specify the unit cell or atomic volumes in the keyblock Latticevalues — a special algorithm has been implemented that accurately estimates it at the pressure of interest, without the need for the user to specify it. This option works well and is available for any calculationType where input volumes are required: 3**, 2D-crystals, 110, 000. You can also use online program http://uspex-team.org/en/uspex/tools/volume_estimation. The users can also input the volumes manually.

(4) If you study molecular crystals under pressure, you might sometimes need to increase the initial volumes somewhat, in order to be able to generate structures by the random symmetric algorithm.

variable splitInto

Meaning: Defines the number of identical subcells or pseudosubcells in the unit cell. If you do not want to use splitting, just use the value 1, or delete the block. Use splitting only for systems with 25–30 atoms/cell.

Default: 1

Format:

% splitInto (number of subcells into which the unit cell is split)
1 2 4
% EndSplitInto

Subcells introduce extra translational (pseudo)symmetry. In addition to this, each subcell can be built using a special space groups algorithm developed by A.R. Oganov and H.T. Stokes and implemented by H.T. Stokes (see Reference ).