# how to compute the angle of a monoclinic crystal

Dear all, I want to know how to compute the angle of a monoclinic crystal by Lammps,which commands can be used?
For example, NiTi alloy of B2 phase（which γ=90°） can be transform to B19’（which γ=98°） under cooling, how to obtain the change of γ?

a discussion of the various box parameters for triclinic cells and their relation is here: https://lammps.sandia.gov/doc/Howto_triclinic.html

please note, that activated processes - like phase transitions - may not happen spontaneously in MD simulations or their occurrence is tied to a (sometimes large) hysteresis. in other words, the transition may happen at different points depending whether you raise temperature or lower it, and it may depend on the rate at which you change it, and it may depend on the system size.

axel.

Dear all, I want to know how to compute the angle of a monoclinic crystal by Lammps,which commands can be used?

In my study, the original crystal is an orthogonal cell(γ=90°) which can transform to a monoclinic cell (γ=98°) under cooling with a certain cooling rate. First, I used “thermo_style cellgamma” command to get the angle, but I found that all the angle values in different time are about 90°. Phase transition can be clearly observed by Ovito, but the simulation cell remains an orthogonal box. I guess there must be something wrong with the command “region box block 0 30 0 30 0 60” which limit all the atoms in a orthogonal box.

So, I modified the command to“region box prism 0 30 0 30 0 60 0 0 0” with a zero tilt factor to ensure the original cell is orthogonal and allow the simulation cell to tilt. In Ovito, we can clearly see that the orthogonal cell suddenly transforms to a monoclinic cell. But the γ is still 90° as well as α and β in log.lammps file.

Is there any problem with using “thermo_style gamma” command to calculate the angle value? I have no way to calculate the angle of a box based on only three sides. I hope you have a suggestion. Thanks again.