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angle_style backmap/harmonic

Syntax

angle_style backmap/harmonic
angle_coeff N at/cg K theta0
  • N -- angle type number
  • at or cg -- weighting mode
    • at: weight = λi × λk (fades in during backmapping)
    • cg: weight = 1 - λi × λk (fades out during backmapping)
  • K -- force constant (energy/radian² units)
  • theta0 -- equilibrium angle (degrees)

Description

A harmonic angle potential scaled by the lambda weight:

\[ E = w \times K (\theta - \theta_0)^2 \]

where \( w \) is computed from the lambda values of the first and last atoms in the angle triplet (i-j-k), using \( \lambda_i \) and \( \lambda_k \).

Note

The equilibrium angle theta0 is specified in degrees in the angle_coeff command but converted to radians internally.

Lambda Weighting

For an angle i-j-k:

  • AT mode (at): \( w = \lambda_i \times \lambda_k \)
  • CG mode (cg): \( w = 1 - \lambda_i \times \lambda_k \)

The central atom j's lambda value is not used in the weight calculation. This is because cross-CG angles typically involve atoms at the boundaries of adjacent beads, and the end atoms' resolution determines the relevance of the interaction.

Example

angle_style backmap/harmonic

# AT cross angle (fades in with lambda)
angle_coeff 1 at 126.67 111.0

In a hybrid setup:

angle_style hybrid harmonic backmap/harmonic

# Intra-bead AT angles (always active)
angle_coeff 1 harmonic 126.67 111.0

# Cross-bead AT angles (lambda-weighted)
angle_coeff 2 backmap/harmonic at 126.67 111.0

Requirements

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

Restart

Angle coefficients (K, theta0, is_cg) are written to and read from restart files.