Request for clarification about the lj/cut potential

LAMMPS LAMMPS Development
1

Hi,

I have a quick question from LAMMPS developers about the lj_cut potential. Assuming that the potential is not shifted and that no long-range tail correction is applied (shift no tail no), is it safe to state that the forces experiences by LJ particles will be exactly identical to those in the shifted potential (shift yes)? In other words, will the only difference between ‘pair_modify shift no tail no’ and ‘pair_modify shift yes’ be the smaller nominal value of the potential energy in the former, while the particles’ trajectories will be the same? This is what I understand from looking at LAMMPS source code, but wanted to make sure that I am not missing anything.

I want to clarify that I am not talking about lj styles that smoothen the potential and the force at the cutoff, just standard shifting (without making the potential or its first derivative continuous).

Regards,

Amir

2

pair_modify shift yes/no only affects the energy and not the forces. Essentially, the energy at the cutoff is pre-computed for each pair of atom types and then subtracted from the pair-wise energy with pair_modify shift yes.

pair_modify tail yes/no affects global energy and global pressure computed with compute pressure. Again, corrections are pre-computed and also assume a homogeneous bulk.

That is not guaranteed. This depends on whether the global energy or global pressure has an influence on the integrator or force computation.

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