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### pair_modify command

Syntax:

    pair_modify keyword values ...

- one or more keyword/value pairs may be listed
- keyword = pair or shift or mix or table or table/disp or tabinner or tabinner/disp or tail or compute

        pair values = sub-style N
          sub-style = sub-style of pair hybrid
          N = which instance of sub-style (only if sub-style is used multiple times)
        mix value = geometric or arithmetic or sixthpower
        shift value = yes or no
        table value = N
          2^N = # of values in table
        table/disp value = N
          2^N = # of values in table
        tabinner value = cutoff
          cutoff = inner cutoff at which to begin table (distance units)
        tabinner/disp value = cutoff
          cutoff = inner cutoff at which to begin table (distance units)
        tail value = yes or no
        compute value = yes or no

Examples:

    pair_modify shift yes mix geometric
    pair_modify tail yes
    pair_modify table 12

Description:

Modify the parameters of the currently defined pair style. Not all parameters are relevant to all pair styles.

If used, the pair keyword must appear first in the list of keywords. It can only be used with the hybrid and hybrid/overlay pair styles. It means that the following parameters will only be modified for the specified sub-style, which must be a sub-style defined by the pair_style hybrid command. If the sub-style is defined multiple times, then an additional numeric argument N must also be specified which is a number from 1 to M where M is the number of times the sub-style was listed in the pair_style hybrid command. The extra number indicates which instance of the sub-style these modifications apply to. Note that if the pair keyword is not used, and the pair style is hybrid or hybrid/overlay, the pair_modify keywords will be applied to all sub-styles.

The mix keyword affects pair coefficients for interactions between atoms of type I and J, when I != J and the coefficients are not explicitly set in the input script. Note that coefficients for I = J must be set explicitly, either in the input script via the "pair_coeff" command or in the "Pair Coeffs" section of the data file. For some pair styles it is not necessary to specify coefficients when I != J, since a "mixing" rule will create them from the I,I and J,J settings. The pair_modify mix value determines what formulas are used to compute the mixed coefficients. In each case, the cutoff distance is mixed the same way as sigma.

Note that not all pair styles support mixing. Also, some mix options are not available for certain pair styles. See the doc page for individual pair styles for those restrictions. Note also that the pair_coeff command also can be to directly set coefficients for a specific I != J pairing, in which case no mixing is performed.

mix geometric

    epsilon_ij = sqrt(epsilon_i * epsilon_j)
    sigma_ij = sqrt(sigma_i * sigma_j)

mix arithmetic

    epsilon_ij = sqrt(epsilon_i * epsilon_j)
    sigma_ij = (sigma_i + sigma_j) / 2

mix sixthpower

    epsilon_ij = (2 * sqrt(epsilon_i*epsilon_j) * sigma_i^3 * sigma_j^3) /
                 (sigma_i^6 + sigma_j^6)
    sigma_ij = ((sigma_i**6 + sigma_j**6) / 2) ^ (1/6)

The shift keyword determines whether a Lennard-Jones potential is shifted at its cutoff to 0.0. If so, this adds an energy term to each pairwise interaction which will be included in the thermodynamic output, but does not affect pair forces or atom trajectories. See the doc page for individual pair styles to see which ones support this option.

The table and table/disp keywords apply to pair styles with a long-range Coulombic term or long-range dispersion term respectively; see the doc page for individual styles to see which potentials support these options. If N is non-zero, a table of length 2^N is pre-computed for forces and energies, which can shrink their computational cost by up to a factor of 2. The table is indexed via a bit-mapping technique (Wolff) and a linear interpolation is performed between adjacent table values. In our experiments with different table styles (lookup, linear, spline), this method typically gave the best performance in terms of speed and accuracy.

The choice of table length is a tradeoff in accuracy versus speed. A larger N yields more accurate force computations, but requires more memory which can slow down the computation due to cache misses. A reasonable value of N is between 8 and 16. The default value of 12 (table of length 4096) gives approximately the same accuracy as the no-table (N = 0) option. For N = 0, forces and energies are computed directly, using a polynomial fit for the needed erfc() function evaluation, which is what earlier versions of LAMMPS did. Values greater than 16 typically slow down the simulation and will not improve accuracy; values from 1 to 8 give unreliable results.

The tabinner and tabinner/disp keywords set an inner cutoff above which the pairwise computation is done by table lookup (if tables are invoked), for the corresponding Coulombic and dispersion tables discussed with the table and table/disp keywords. The smaller the cutoff is set, the less accurate the table becomes (for a given number of table values), which can require use of larger tables. The default cutoff value is sqrt(2.0) distance units which means nearly all pairwise interactions are computed via table lookup for simulations with "real" units, but some close pairs may be computed directly (non-table) for simulations with "lj" units.

When the tail keyword is set to yes, certain pair styles will add a long-range VanderWaals tail "correction" to the energy and pressure. See the doc page for individual styles to see which support this option. These corrections are included in the calculation and printing of thermodynamic quantities (see the thermo_style command). Their effect will also be included in constant NPT or NPH simulations where the pressure influences the simulation box dimensions (e.g. the fix npt and fix nph commands). The formulas used for the long-range corrections come from equation 5 of (Sun).

Several assumptions are inherent in using tail corrections, including the following:

- The simulated system is a 3d bulk homogeneous liquid. This option should not be used for systems that are non-liquid, 2d, have a slab geometry (only 2d periodic), or inhomogeneous.
- G(r), the radial distribution function (rdf), is unity beyond the cutoff, so a fairly large cutoff should be used (i.e. 2.5 sigma for an LJ fluid), and it is probably a good idea to verify this assumption by checking the rdf. The rdf is not exactly unity beyond the cutoff for each pair of interaction types, so the tail correction is necessarily an approximation.
- Thermophysical properties obtained from calculations with this option enabled will not be thermodynamically consistent with the truncated force-field that was used. In other words, atoms do not feel any LJ pair interactions beyond the cutoff, but the energy and pressure reported by the simulation include an estimated contribution from those interactions.

The compute keyword allows pairwise computations to be turned off, even though a pair_style is defined. This is not useful for running a real simulation, but can be useful for debugging purposes or for performing a rerun simulation, when you only wish to compute partial forces that do not include the pairwise contribution.

Two examples are as follows. First, this option allows you to perform a simulation with pair_style hybrid with only a subset of the hybrid sub-styles enabled. Second, this option allows you to perform a simulation with only long-range interactions but no short-range pairwise interactions. Doing this by simply not defining a pair style will not work, because the kspace_style command requires a Kspace-compatible pair style be defined.

Restrictions: none

You cannot use shift yes with tail yes, since those are conflicting options. You cannot use tail yes with 2d simulations.

Related commands:

pair_style, pair_coeff, thermo_style

Default:

The option defaults are mix = geometric, shift = no, table = 12, tabinner = sqrt(2.0), tail = no, and compute = yes.

Note that some pair styles perform mixing, but only a certain style of mixing. See the doc pages for individual pair styles for details.

(Wolff) Wolff and Rudd, Comp Phys Comm, 120, 200-32 (1999).

(Sun) Sun, J Phys Chem B, 102, 7338-7364 (1998).