c2x and Forces

C2x can read forces from Castep's .check files, and Quantum Espresso's XML files.

Again using XCrysden and Castep, and using as an input an 8-atom silicon cell with one atom significantly displaced:

si8f.cell

%BLOCK LATTICE_CART
    5.46    0.00    0.00
    0.00    5.46    0.00
    0.00    0.00    5.46
%ENDBLOCK LATTICE_CART

%BLOCK POSITIONS_FRAC
    Si    0.00    0.00    0.00
    Si    0.00    0.50    0.50
    Si    0.50    0.00    0.50
    Si    0.50    0.50    0.00
    Si    0.75    0.30    0.75
    Si    0.25    0.25    0.25
    Si    0.25    0.75    0.75
    Si    0.75    0.75    0.25
%ENDBLOCK POSITIONS_FRAC

symmetry_generate

kpoints_mp_grid 2 2 2
kpoints_mp_offset 0.125 0.125 0.125

Forces are automatically included in c2x's output when writing XSF files, so, to produce the following, it was merely necessary to:

Increase the "chemical connectivity factor" under "atomic radius" to 1.1.
Turn "crystal cells" on.
Turn "Forces" on ("Display" menu).
Set the "length factor" (Modify, Force Settings) to 100.

Silicon and forces

Note that though there are eight atoms in the unit cell, XCrysden shows 18. This is because it shows atoms on the edges of the unit cell on all possible edges. What is drawn is not a genuine repeat unit of the crystal, and will not necessarily have the sum of the forces begin zero. If you wish the real repeat unit, choose "Display, Unit of Repetition, Translational asymmetric unit".

(Note that versions of XCrysDen prior to 1.5beta have a bug in which forces are scaled quadratically, not linearly.)