Hello,
I am setting up a simulation with atom_style hybrid dipole sphere. The version of LAMMPS that I am using is 29 Aug 2024 - Update 1. I am bit confused about how masses are set (I want to a have combination of finite sphere particles and point particles in the simulation). Atoms are created using create_atoms command. I went through the documentation, previous discussions on the forum, and ran a few tests and concluded following points:
- The diameter and density are not explicitly set for point particles, diameter is set to a specific value for spherical particles and only per-type masses are set for all the atoms. In this case, LAMMPS will calculate the mass of atoms (shown in dump file) using default value of diameter and density i.e., 1 for point particles. However, the output data file shows the correct masses set by “mass” command. For spherical particles, the diameter will be the set value but the mass shown in the dump file will still be ~0.5236 (calculated using diameter and density value of 1), and the density will be calculated using this mass (~0.5236) and set diameter. This simulation is not correct.
- The diameter and density are not explicitly set for point particles, diameter is set to a specific value for spherical particles and both per-type and per-atom masses are set for all the atoms. LAMMPS will use the per-atom masses set by the user. In this case, the diameter will be 1 for point particles i.e., default set by create_atoms command but the density for all the atoms will be calculated by using mass and diameter value and will not have the default value i.e., 1.
- The diameter is set to 0 for point particles, diameter is set to a specific value for spherical particles and both per-type and per-atom masses are set for all the atoms. LAMMPS will use the per-atom masses set by the user. In this case, the density will be equal to the per-atom mass for point particles and not the default value i.e., 1.
I wanted to ask if my understanding is correct? Both cases 2 and 3 also provide the same thermo output which indicates that for point particles where specific fix developed for spherical particles such as nve/sphere are not used and pair style defined by lj/cut is used, the diameter and density do not matter as long as per-atom type masses are explicitly set. The LAMMPS documentation is confusing because according to it, mass is calculated from density and volume (mentioned in set and read_data commands) which does not agree with cases 2 and 3 defined above.
Moreover, LAMMPS documentation also mentions that " If you define a hybrid atom style which includes one (or more) sub-styles which require per-type mass and one (or more) sub-styles which require per-atom mass, then you must define both. However, in this case the per-type mass will be ignored; only the per-atom mass will be used by LAMMPS." A warning is also printed in the output file stating the same. However, for cases 2 and 3, if I don’t set the per-type masses and only set the per-atom masses, thermo output remains the same. However, masses are printed as 0 in the output data file while correct masses are printed in the dump file. How can this effect the simulation?
Thank you for the time, help and guidance.