an the N^2 style
skin must be large enough that all atoms needed for bond
  interactions are also acquired by interprocessor communication
last parameter incurs extra checking and communication to test against
  skin thickness, but may mean neighbor list is created less often
when rRESPA is run, the 3rd and 4th parameters refer to the
  nonbond (short-range) timestepping
normally this command should be used before the data or restart file is read,
  since the skin distance is used to estimate memory needed 