LAMMPS output with pair_style srp

Dear LAMMPS Community,

I am a bit confused with the LAMMPS output when pair_style srp is used for DPD-polymer systems. Here are the settings in input script.

special_bonds lj 1.0 1.0 1.0

bond_style fene

The LJ interactions in FENE bond_style are turned off

to allow only DPD interactions between polymer beads

bond_coeff 1 40.0 5.0 0.0 0.0

pair_style hybrid dpd {tset} 1.0 (v_seed) srp 1.0 1 mid

The lammps output has:

Total # of neighbors = 0
Ave neighs/atom = 0.0000000
Ave special neighs/atom = 1.5818182
Neighbor list builds = 5504
Dangerous builds = 0

I don’t understand why “Total # of neighbors = 0” and “Ave neighs/atom = 0.0000000”. This is also the case with the example given in examples/PACKAGES/srp.

Is this because, with srp style bond particles are added before the simulation starts and deleted when simulation ends?? Not very clear to me.

Thanks you,


The answer is in the neighbor list summary (here the one from the srp example log):

  3 neighbor lists, perpetual/occasional/extra = 3 0 0
  (1) pair dpd, perpetual, skip from (3)
      attributes: half, newton on
      pair build: skip
      stencil: none
      bin: none
  (2) pair srp, perpetual, skip from (3)
      attributes: half, newton on
      pair build: skip
      stencil: none
      bin: none
  (3) neighbor class addition, perpetual
      attributes: half, newton on
      pair build: half/bin/newton
      stencil: half/bin/3d
      bin: standard

The neighbor list statistics output code looks for either a full or half neighbor list request that is not a “skip” list. But there are only two skip lists available and an internal list that gets added to be the “master” list from which the skip lists can be created but for which no explicit request exists.
So there is no matching list and thus the statistics are zero.

This is a somewhat approximate statistic anyway, since it will only print results from a single neighbor list, even if there are multiple.

Yes Dr. Kohlmeyer, I saw that too, but could not decipher that neighbor list summary. So, this is nothing to worry about?

By the way, the physics seems to be correct. But, this output bothered me.