Order of coefficients in pair_coeff for pair_lj

Dear all,

is there any documentation that lists the sequence of coefficients in the pair_coeff for the lj pair_style family? On the pair_coeff page, it specifies that this information should be on the respective pair_style page, but for the life of me I cannot find it on http://lammps.sandia.gov/doc/pair_lj.html

Unless I’m struck with blindness, could one please highlight this IMHO important information near the top of the page (as it is for example done on the pair_gauss page)?

Thanks,

Peter

Dear all,

is there any documentation that lists the sequence of coefficients in the
pair_coeff for the lj pair_style family? On the pair_coeff page, it
specifies that this information should be on the respective pair_style
page, but for the life of me I cannot find it on
http://lammps.sandia.gov/doc/pair_lj.html

Unless I’m struck with blindness, could one please highlight this IMHO
important information near the top of the page (as it is for example done
on the pair_gauss page)?

​this is from the page that documents lj/cut:

​For all of the lj/cut pair styles, the following coefficients must be
defined for each pair of atoms types via the pair_coeff command as in the
examples above, or in the data file or restart files read by the read_data
or read_restart commands, or by mixing as described below:

epsilon (energy units)
sigma (distance units)
cutoff1 (distance units)
cutoff2 (distance units)
Note that sigma is defined in the LJ formula as the zero-crossing distance
for the potential, not as the energy minimum at 2^(1/6) sigma.

The latter 2 coefficients are optional. If not specified, the global LJ and
Coulombic cutoffs specified in the pair_style command are used. If only one
cutoff is specified, it is used as the cutoff for both LJ and Coulombic
interactions for this type pair. If both coefficients are specified, they
are used as the LJ and Coulombic cutoffs for this type pair. You cannot
specify 2 cutoffs for style lj/cut, since it has no Coulombic terms.

​that should tell you what you are looking for. the problem​ is, that this
page has become very convoluted over time and should be split into multiple
pages. it follows the same order as other pair style documentations, but
since it first explains a long list of pair style options, you'll have to
scroll down a bit to see the pair_coeff documentation part.

apart from that, i still have to see the MD code that enters LJ parameters
*not* in the order epsilon, sigma.

axel.

Hi Peter,

Somewhere towards the end of the very same page you linked:

For all of the lj/cut pair styles, the following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands, or by mixing as described below:

  • epsilon (energy units)
  • sigma (distance units)
  • cutoff1 (distance units)
  • cutoff2 (distance units)
    and, at least from my experience, that’s the order in which they have to appear in the input file.

I hope this helps.
Best wishes,
Nicola

Thanks Axel and Nicola,

(off-list to reduce unnecessary traffic - not expecting any further reply)

it was indeed blindness on my side. It is now so glaringly obvious, that I can’t explain how I scrolled by that several times without seeing.

Many thanks and apologies,

Peter