Syntax:
pair_modify keyword value ...
shift value = yes or no mix value = geometric or arithmetic or sixthpower table value = N 2^N = # of values in table
Examples:
pair_modify shift yes pair_modify mix arithmetic pair_modify table 12
Description:
Modify the parameters of the currently defined pair style. Not all parameters are relevant to all pair styles.
The shift keyword determines whether the 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 printed in the thermodynamic output, but does not affect atom dynamics (forces). Pair styles that are already 0.0 at their cutoff such as lj/charmm/coul/charmm are not affected by this setting.
The mix keyword affects how Lennard-Jones coefficients for epsilon and sigma are generated for interactions between atoms of type I and J, when I != J. (I = J coefficients are set explicitly in the data file or input script.) The pair_coeff command can be used in the input script to specify epilon/sigma for a specific I,J pairing, which overrides the setting of the mix keyword. In each case, the LJ cutoff is mixed the same way as sigma.
These are the formulas used by the 3 mix options:
geometric
epsilon_ij = sqrt(epsilon_i * epsilon_j) sigma_ij = sqrt(sigma_i * sigma_j)
arithmetic
epsilon_ij = sqrt(epsilon_i * epsilon_j) sigma_ij = (sigma_i + sigma_j) / 2
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)
Style soft only uses a pre-factor coefficient, which is always mixed geometrically, regardless of the mix setting. The charmm styles are always mixed arithmetically, regardless of the mix setting. The class2 styles are always mixed as a sixthpower, regardless of the mix setting, except that the cutoff is mixed according to the mix setting. Style lj/expand always mixes its delta coefficient using the rule
delta_ij = (delta_i + delta_j) / 2
The table keyword applies to pair styles with a long-range Coulombic term (lj/cut/coul/long and lj/charmm/coul/long). 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 table is only used for pair distances >= sqrt(2.0) which means this option will have more effect on simulations in "real" units than those in "lj" units.
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.
Restrictions: none
Related commands:
Default:
The option defaults are shift = no, mix = arithmetic (for lj/charmm pair styles), mix = geometric (for other pair styles), and table = 12.
(Wolff) Wolff and Rudd, Comp Phys Comm, 120, 200-32 (1999).