diff --git a/doc/src/pair_hybrid.rst b/doc/src/pair_hybrid.rst index 972c30c36f..b8bbbd2685 100644 --- a/doc/src/pair_hybrid.rst +++ b/doc/src/pair_hybrid.rst @@ -198,8 +198,8 @@ same: Coefficients must be defined for each pair of atoms types via the :doc:`pair_coeff ` command as described above, or in the -data file read by the :doc:`read_data ` commands, or by -mixing as described below. +PairCoeffs or PairIJCoeffs section of the data file read by the +:doc:`read_data ` command, or by mixing as described below. For all of the *hybrid*, *hybrid/overlay*, and *hybrid/scaled* styles, every atom type pair I,J (where I <= J) must be assigned to at least one @@ -208,14 +208,21 @@ examples above, or in the data file read by the :doc:`read_data `, or by mixing as described below. Also all sub-styles must be used at least once in a :doc:`pair_coeff ` command. +.. note:: + + LAMMPS never performs mixing of parameters from different sub-styles, + **even** if they use the same type of coefficients, e.g. contain + a Lennard-Jones potential variant. Those parameters must be provided + explicitly. + If you want there to be no interactions between a particular pair of -atom types, you have 3 choices. You can assign the type pair to some -sub-style and use the :doc:`neigh_modify exclude type ` +atom types, you have 3 choices. You can assign the pair of atom types +to some sub-style and use the :doc:`neigh_modify exclude type ` command. You can assign it to some sub-style and set the coefficients so that there is effectively no interaction (e.g. epsilon = 0.0 in a LJ potential). Or, for *hybrid*, *hybrid/overlay*, or *hybrid/scaled* simulations, you can use this form of the pair_coeff command in your -input script: +input script or the "PairIJ Coeffs" section of your data file: .. code-block:: LAMMPS @@ -238,19 +245,20 @@ styles with different requirements. ---------- -Different force fields (e.g. CHARMM vs AMBER) may have different rules -for applying weightings that change the strength of pairwise -interactions between pairs of atoms that are also 1-2, 1-3, and 1-4 -neighbors in the molecular bond topology, as normally set by the -:doc:`special_bonds ` command. Different weights can be -assigned to different pair hybrid sub-styles via the :doc:`pair_modify -special ` command. This allows multiple force fields to be -used in a model of a hybrid system, however, there is no consistent -approach to determine parameters automatically for the interactions -between the two force fields, this is only recommended when particles +Different force fields (e.g. CHARMM vs. AMBER) may have different rules +for applying exclusions or wheights that change the strength of pairwise +non-bonded interactions between pairs of atoms that are also 1-2, 1-3, +and 1-4 neighbors in the molecular bond topology. This is normally a +global setting defined the :doc:`special_bonds ` command. +However, different weights can be assigned to different hybrid +sub-styles via the :doc:`pair_modify special ` command. +This allows multiple force fields to be used in a model of a hybrid +system, however, there is no consistent approach to determine parameters +automatically for the interactions **between** atoms of the two force +fields, thus this approach this is only recommended when particles described by the different force fields do not mix. -Here is an example for mixing CHARMM and AMBER: The global *amber* +Here is an example for combining CHARMM and AMBER: The global *amber* setting sets the 1-4 interactions to non-zero scaling factors and then overrides them with 0.0 only for CHARMM: @@ -260,7 +268,7 @@ then overrides them with 0.0 only for CHARMM: pair_style hybrid lj/charmm/coul/long 8.0 10.0 lj/cut/coul/long 10.0 pair_modify pair lj/charmm/coul/long special lj/coul 0.0 0.0 0.0 -The this input achieves the same effect: +This input achieves the same effect: .. code-block:: LAMMPS @@ -270,9 +278,9 @@ The this input achieves the same effect: pair_modify pair lj/cut/coul/long special coul 0.0 0.0 0.83333333 pair_modify pair lj/charmm/coul/long special lj/coul 0.0 0.0 0.0 -Here is an example for mixing Tersoff with OPLS/AA based on -a data file that defines bonds for all atoms where for the -Tersoff part of the system the force constants for the bonded +Here is an example for combining Tersoff with OPLS/AA based on +a data file that defines bonds for all atoms where - for the +Tersoff part of the system - the force constants for the bonded interactions have been set to 0. Note the global settings are effectively *lj/coul 0.0 0.0 0.5* as required for OPLS/AA: