Lattice command, scale factor, lattice parameter and conversion between lattice and distance units

LAMMPS LAMMPS Beginners
1

Hi everyone! I’ve been trying to understand something that is probably really simple but I’ve managed to hopelessly confuse myself by overthing it. Please bear with me and my following stream of consciousness:

When using the lattice command I’m setting up a set of points in space. This is done by specifying a unit cell, in my case FCC with a scale factor, or lattice parameter, of 3.615 Angstrom. I also use the orient keyword to obtain a 111 surface. Then, throughout my script I use lattice units. After minimization I will be able to determine the true lattice parameter since the lattice parameter given in the lattice command is an approximation. So, the lattice parameter will effectively change but as far as I’m aware the scale factor in the lattice command remains unchanged. At the end of the script I convert some values from lattice units to distance units using the xlat, ylat and zlat parameters. But these parameters are determined from the lattice command and the “approximate” lattice constant I provide there. Since the lattice constant from the lattice command is only a scale factor, I assume that my conversions should be sound, but what confuses me is that the scale factor does have distance units associated with it. I’m almost sure that using xlat, ylat and zlat for these conversions should work, but I just can’t wrap my mind around the logic of it.

I hope I was able to convey the problem. I’m just struggling to visualize it and therefore verbalizing it is also challenging. I hope I don’t seem too ignorant.

Thank you for taking the time to consider my question!

2

Yes, this is like using ruled paper, only in 3d.

Provided the minimization finds the true absolute minimum geometry and box dimensions and doesn’t get stuck in some other, less deep minimum. Specifically on shallow potential hypersurfaces, this can happen easily.

You know how many times you have replicated the unit cell in x, y, and z and you have access to the box dimensions in x, y, and z, so you can compute from that the updated xlat, ylat, and zlat values.
You can double check by doing this computation also before you optimize the box volume. In fact, you can define those computations as equal style variables and monitor their values as thermo output throughout the calculations.

3

I am going to do what you suggested. It actually makes so much sense that I feel silly for not thinking of it myself. I think I understand the whole concept better now as well. I’m super excited to see what my new results will yield! Thank you so much for your advice, I appreciate it so much.