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Check2xsf and Forces

Again using XCrysden, 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 check2xsf's output when writing XSF files, so, to produce the following, it was merely necessary to:

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 too that versions of XCrysDen prior to 1.5beta have a bug in which forces are scaled quadratically, not linearly. If you wish to test for this bug, try viewing the XSF file

ATOMS
0 0 0 0 0.00 0 0
0 0 1 0 0.01 0 0
0 0 2 0 0.02 0 0
0 0 3 0 0.03 0 0
0 0 4 0 0.04 0 0
0 0 5 0 0.05 0 0
0 0 6 0 0.06 0 0
0 0 7 0 0.07 0 0
The force arrows should increase in length linearly, not quadratically, and the length factor should need reducing to about 30. The first image below is incorrect, the second correct.

Incorrect force lengths Correct force lengths