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:

$\triangleright$ 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 make reasonable estimates at any pressure.

Format:

% Latticevalues 125.00 % Endvalues

Notes: (1) This volume is only used as an initial guess to speed up structure relaxation and does not affect the results, because 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 3 $\times$ 3 matrix (calculationType = 300/310) or 2 $\times$ 2 matrix (-200) 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

Alternatively, you can specify unit cell parameters just by listing a, b, c, $\alpha$ , $\beta$ , and $\gamma$ 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 $\beta$ (standard setting).

For 2D crystal (calculationType = -200), you just need cell parameters a, b, and $\gamma$ .

% Latticevalues 10.1 8.4 90.0 % Endvalues

(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 https://uspex-team.org/online_utilities/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 initial random structures.