Xiandong,

It defines entries in a shape matrix, as defined by Parrinello and Rahman in for instance J. Appl. Phys. 1982, 52, 7182. Shape matrices are based on crystallographic vectors a, b, and c, where h = {a, b, c}. We adhere to the convention a = (xx, 0, 0)^t, b = (yx, yy, 0)^t, and c = (zx, zy, zz)^t, which gives for the shape matrix h

xx yx zx

0 yy zy

0 0 zz

This can describe any crystallographic geometry. In the example below I used

xx = 5 - (-5) = 10

yx = 1

yy = 5 - (-5) = 10

zx = 0

zy = 0

zz = 5 - (-5) = 10

To get LAMMPS to understand this geometry, you will need to add a line currently containing xy xz yz in your data file as outlined in the example below.

Pieter