I read it manual section 6.13 and 6.12 in detail but i have 216 atoms positions and three lattice vector get by ab initio calculation i do it manuallly math but i canot understand how to find tilt factor please help me if possible to define triclinic simmulation box

The first sentences of section 6.12 say:

The parallelepiped has its “origin” at (xlo,ylo,zlo) and is defined by 3 edge vectors starting from the origin given by **a** = (xhi-xlo,0,0); **b** = (xy,yhi-ylo,0); **c** = (xz,yz,zhi-zlo). *xy,xz,yz* can be 0.0 or positive or negative values and are called “tilt factors” because they are the amount of displacement applied to faces of an originally orthogonal box to transform it into the parallelepiped.

If you have your lattice vectors a,b,c and they follow the LAMMPS convention, then that tells you what the

tilt factors are. If they don’t follow the LAMMPS convention, you have to rotate

them to an a’,b’,c’ that do, using the formulas below.

Steve