Definition of atom types designating the chemical symbol provided in Compute XRD documentation

Dear everyone,

I am new to LAMMPS and trying to perform the XRD analysis of a polymer material. Is there any documentation where i can get the definition of the atom types that is designating the chemical symbols to be provided for the compute XRD command. If there is any, I would be grateful for the reference.

Please see the source code: lammps/compute_xrd_consts.h at 584943fc928351bc29f41a132aee3586e0a2286a · lammps/lammps · GitHub
And the corresponding reference in the documentation: compute xrd command — LAMMPS documentation

Thank you sir for your reply. At first I misunderstood the symbols of the elements with atom types definition in force fields. But now it is clear. However, I would be grateful if you can throw some light in the below problem that i am facing while executing,

I am trying an simple example with methane (CH4) to understand the compute xrd command.:-

  1. I have build the initial data files using moltemplate
  2. I have run initial minimize (Conjugate gradient), reorientation of molecule by NVT dynamics and followed by NPT dynamics to restructure the simulation box, desired temperature, pressure to complete the equilibration process as shown in examples of moltemplate
  3. After this, I have run a small NVT dynamics and followed it to compute the XRD by the compute xrd command as below,

compute 1 all xrd 1.541838 C H 2Theta 0.087 0.87 c 1 1 1 LP 1 echo

fix fxnvt all nvt temp 260.0 260.0 500.0 tchain 1

fix 1 all ave/histo/weight 1 1 1 0.087 0.87 250 c_1[1] c_1[2] mode vector file Rad2Theta.xrd
run 10000

ERROR: Illegal Compute XRD Command (src/USER-DIFFRACTION/compute_xrd.cpp:67)

I have installed the LAMMPS stable version - Pre-built Ubuntu Linux executables and have checked that USER-DIFFRACTION is already installed package. So, Where am I going wrong in the compute xrd command ?

“Illegal XXX command” errors are usually syntax errors, most frequently an incorrect number of arguments. You have to check with the documentation.