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### fix species command

Syntax:

    fix ID group-ID species Nevery Nrepeat Nfreq filename keyword value ...

- ID, group-ID are documented in fix command
- species = style name of this command
- Nevery = sample bond-order every this many timesteps
- Nrepeat = # of bond-order samples used for calculating averages
- Nfreq = calculate average bond-order every this many timesteps
- filename = name of output file
- zero or more keyword/value pairs may be appended
- keyword = cutoff or element

        cutoff value = I J Cutoff
          I, J = atom types
          Cutoff = Bond-order cutoff value for this pair of atom types:pre
        element value = Element1, Element2, ...

Examples:

    fix 1 all species 10 10 100 species.out
    fix 1 all species 1 2 20 species.out cutoff 1 1 0.40 cutoff 1 2 0.55
    fix 1 all species 10 10 100 species.out element Au O H

Description:

Write out the chemical species information computed by the ReaxFF potential specified by pair_style reax/c. Bond-order values (either averaged or instantaneous, depending on value of Nrepeat) are used to determine chemical bonds. Every Nfreq timesteps, chemical species information is written to filename as a two lines. The first line is a header containing labels. The second line consists of the following: timestep, total number of molecules, total number of distinct species, number of molecules of each species. The chemical formula of each species is given in the first line.

Optional keyword cutoff can be assigned to change the minimum bond-order values used in identifying chemical bonds between pairs of atoms. Bond-order cutoffs should be carefully chosen, as bond-order cutoffs that are too small may include too many bonds (which will result in an error), while too-large cutoffs will result in fragmented molecules. The default cutoff of 0.3 usually gives good estimate.

Optional keyword element can be used to specify the chemical symbol printed for each LAMMPS atom type. The number of symbols must match the number of LAMMPS atom types and each symbol must consist of 1 or 2 alphanumeric characters. Normally, these symbols should be chosen to match the chemical identity of each LAMMPS atom type, as specified using the reax/c pair_coeff command and the ReaxFF force field file.

The Nevery, Nrepeat, and Nfreq arguments specify on what timesteps the bond-order values are sampled to get the average bond order. The species analysis is performed using the average bond-order on timesteps that are a multiple of Nfreq. The average is over Nrepeat bond-order samples, computed in the preceding portion of the simulation every Nevery timesteps. Nfreq must be a multiple of Nevery and Nevery must be non-zero even if Nrepeat is 1. Also, the timesteps contributing to the average bond-order cannot overlap, i.e. Nfreq > (Nrepeat-1)*Nevery is required.

For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then bond-order values on timesteps 90,92,94,96,98,100 will be used to compute the average bond-order for the species analysis output on timestep 100.

Restart, fix_modify, output, run start/stop, minimize info:

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Restrictions:

The fix species currently only works with pair_style reax/c and it requires that the pair_style reax/c be invoked. This fix is part of the USER-REAXC package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

It should be possible to extend it to other reactive pair_styles (such as rebo, airebo, comb, and bop), but this has not yet been done.

Related commands:

pair_style reax/c, fix reax/bonds

Default:

The default values for bond-order cutoffs are 0.3 for all I-J pairs. The default element symbols are C, H, O, N.