Elastic Modulus calculation for RDX using rxff based on example elastic modulus script

Dear Lammps users,

I am trying to calculate the elastic modulus for rdx using lammps. And I am using the example script which is provided in the example section of lammps.

I am encountering a few problems in the calculation of elastic modulus.

The problem is I am a factor of 3 off in the values of C11, C22 and C33.

Based on the schedule and output I do not find any problems as the energy values and temperatures all converge to a proper value. I am using a perturbation of 1% in any direction to calculate the stress and strain values, which is same as that in the schedule given in lammps.

I am attaching the scripts I am using for calculating the elastic modulus.


property.mod (372 Bytes)

pot.mod (95 Bytes)

init.mod (1.54 KB)

in.elastic (6.54 KB)

equlli_2.mod (537 Bytes)

equlli_1.mod (359 Bytes)

displace.mod (3.3 KB)

constant.mod (1.63 KB)


Did you have charges on the C,H,N,O atoms within the RDX molecular crystal? If yes, did you carry out an Ewald summation?

Best Regards



Ankit is using ReaxFF so it doesn’t need a kspace_style.


  1. What type of ReaxFF description is “ffield.reax” defined in your pot.mod? Is it the one for RDX?

  2. Check what was defined in the control file lmp_control – it might work just fine with “NULL”.

  3. What reference values are you comparing your values to?