Spring Constant in core-shell harmonic bond

Dear all,

I am working on Dec 2018 version of LAMMPS. I am doing NVT MD
simulation of ionic solid supercell. In the system I require to use bond_style harmonic, so in pair_coefficient I need to mention spring constant, in my case it is k=67.3564. Referring Lammps doc for “bond_style harmonic” that mentions “Note that the usual 1/2 factor is included in K.”, I used an argument in pair_coefficients k=33.6892 (i.e. half of value). With this k value the dynamics is failing.

While trying to find the cause, I spent lot of time going through other parameters and commands which might be possibly causing failed dynamics, no positive effect of it. Later while referring to an example directory of ‘coreshell’ in lammps, I noticed the spring constant value is not halved and written as it is as in its source literature(i.e.: Mitchell, Fincham, J Phys Condensed Matter, 5, 1031-1038). I ran a simulation of my system with k=67.3564 (not halved) and the simulation worked!!.

Now this is confusing me, what is correct? what should I keep as a spring constant value? Did I misinterpret the information given in document corresponding to bond_style harmonic? Whether the lammps example of NaCl coreshell model has mentioned wrong input? Please clarify. My input code is attached below. Please let me know if any more information required. Also, any other suggestions regarding the input script are most welcome.

Thank you,
Saurabh S

in.lfp (1.73 KB)

Dear all,

I am working on Dec 2018 version of LAMMPS. I am doing NVT MD
simulation of ionic solid supercell. In the system I require to use bond_style harmonic, so in pair_coefficient I need to mention spring constant, in my case it is k=67.3564. Referring Lammps doc for “bond_style harmonic” that mentions “Note that the usual 1/2 factor is included in K.”, I used an argument in pair_coefficients k=33.6892 (i.e. half of value). With this k value the dynamics is failing.

While trying to find the cause, I spent lot of time going through other parameters and commands which might be possibly causing failed dynamics, no positive effect of it. Later while referring to an example directory of ‘coreshell’ in lammps, I noticed the spring constant value is not halved and written as it is as in its source literature(i.e.: Mitchell, Fincham, J Phys Condensed Matter, 5, 1031-1038). I ran a simulation of my system with k=67.3564 (not halved) and the simulation worked!!.

Now this is confusing me, what is correct? what should I keep as a spring constant value?

you have to compare against what functional form for the harmonic potential is used in the paper you are taking the parameters from. if they use the same expression as LAMMPS, you must not apply a factor of 1/2, since it is already included.

axel.