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pair_style backmap

Syntax

pair_style backmap cut_at at_style at_args ... cut_cg cg_style cg_args ...
  • cut_at -- cutoff for AT sub-style (distance units)
  • at_style -- name of the AT pair sub-style (e.g., lj/cut/coul/cut)
  • at_args -- arguments passed to the AT sub-style
  • cut_cg -- cutoff for CG sub-style (distance units)
  • cg_style -- name of the CG pair sub-style (e.g., table)
  • cg_args -- arguments passed to the CG sub-style

Description

pair_style backmap is a hybrid-like pair style that delegates force computation to two sub-styles (AT and CG) and weights the results by the current lambda values of the interacting atoms.

For each pair of atoms i and j:

  • AT-type pairs (tagged atomistic in pair_coeff): force is weighted by \( w_\text{AT} = \lambda_i \times \lambda_j \)
  • CG-type pairs (tagged cg in pair_coeff): force is weighted by \( w_\text{CG} = 1 - \lambda_i \times \lambda_j \)
  • None pairs (tagged none): no interaction

Both force and energy are scaled by the weight factor. Interactions with negligible weight (< 10-10) are skipped for efficiency.

pair_coeff

pair_coeff I J atomistic at_args ...
pair_coeff I J cg cg_args ...
pair_coeff I J none
  • I, J -- atom type pair (1-indexed, can use * for ranges)
  • atomistic -- forward coefficients to the AT sub-style, weight by λλ
  • cg -- forward coefficients to the CG sub-style, weight by 1 - λλ
  • none -- no interaction between these types

The AT and CG arguments after the keyword are passed directly to the respective sub-style's pair_coeff command.

Requirements

This pair style requires fix backmap to be defined. It reads per-atom lambda values from the fix at each timestep.

Example

# AT sub-style: LJ + Coulomb with 14 Å cutoff
# CG sub-style: tabulated with 14 Å cutoff
pair_style backmap 14.0 lj/cut/coul/cut 14.0 9.0 14.0 table linear 1000

# CG-CG interactions (fade out with lambda)
pair_coeff 1 1 cg 0.0 0.0
pair_coeff 1 2 cg 0.0 0.0
pair_coeff 2 2 cg 0.0 0.0

# Cross-resolution interactions (no interaction)
pair_coeff 1 3 none
pair_coeff 1 4 none
pair_coeff 2 3 none
pair_coeff 2 4 none

# AT-AT interactions (fade in with lambda)
pair_coeff 3 3 atomistic 0.207266 3.748000
pair_coeff 3 4 atomistic 0.156387 3.826500
pair_coeff 4 4 atomistic 0.117997 3.905000

In this example:

  • Types 1-2 are CG beads; they interact via tabulated CG potentials
  • Types 3-4 are AT atoms; they interact via LJ + Coulomb
  • CG-AT cross pairs have no direct interaction (forces are distributed through fix backmap)

Restart

Cutoff values are written to restart files. Sub-style parameters must be re-specified after a restart.