[lammps-users] no pair-interactions with TIP3?

Dear All...

I have a box of TIP3 water...but E_pair=0 thoughout the simulation (even if I
pack the water closely and extend the force cutoffs). I take that to mean
that the LJ/Coulumb sites aren't interacting; however, the logfile gives a
non-zero number for "Pair time". What gives? Input file is below...


Looks like the problem is in these lines:

pair_coeff 1 1 0.046 0.400014 units real
pair_coeff 2 2 0.1521 3.15057 units real

Get rid of the "units real" part. If that doesn't
work, could you send your multi.data file as well? If
it is large, you probably want to zip it and not send
it to the list (just to [email protected]...).


The problem with adding "units real" to these lines, is that
LAMMPS allows there to be 4 coeffs (in addition to i,j) for
this potential. When there are 4, it considers the last
2 to be a LJ and Coulombic cutoff distance. Since these
are words, not numbers, they likely get converted to a value
of 0.0.

So these lines set the cutoff for 1-1 and 2-2 interactions
to 0 and thus there are no interactions.


Should have added that you could append "# units real" to the
pair_coeff lines, then it's just a comment that will be stripped.