Repulsive Walls¶
Three repulsive-wall operators confine or impact the system with a planar, cylindrical, or spherical barrier. All three share the same repulsive power-law form and are fed into force computation like any other potential, via compute_force. Each has a companion move_* operator that computes the wall's time-varying position/size — this is what gives a "piston" its motion.
Repulsive wall equations
For a particle at signed distance \(d\) from the wall geometry (plane/cylinder axis/sphere center), with \(|d| \le\) cutoff, the energy contribution is:
\[
E_p \mathrel{+}= \varepsilon \left(1 - \frac{\text{cutoff}}{d}\right)^{\text{exponent}}
\]
and the resulting force:
\[
\mathbf{f} = -\varepsilon \cdot \text{exponent} \cdot \frac{\text{cutoff}}{d^2} \left(1 - \frac{\text{cutoff}}{d}\right)^{\text{exponent}-1} \hat{\mathbf{n}}
\]
Particles just inside the wall are pushed further inward, particles just outside are pushed further outward — a thin repulsive shell/membrane, not a one-sided barrier.
Planar Wall¶
plane_wall¶
Parameters
normal: Vec3d, default [1,0,0] # Unit normal of the plane.
offset: float, default 0.0 # Signed distance of the plane from the origin along normal.
cutoff: float, required # Interaction range from the plane.
exponent: int, default 12 # Power-law exponent.
epsilon: float, default 1.0e-19 J # Energy scale.
Typically fed by move_plane_wall rather than set directly, so the wall can move over time.
Usage example
left_wall:
- plane_wall: { normal: [1.0,0.0,0.0], offset: 17.0 ang, cutoff: 2.2 ang, epsilon: 1.0e-19 J }
right_wall:
- plane_wall: { normal: [-1.0,0.0,0.0], offset: -83.0 ang, cutoff: 2.2 ang, epsilon: 1.0e-19 J }
+compute_force: [ left_wall, right_wall ]
move_plane_wall¶
Syntax
move_plane_wall:
init_normal: <Vec3d>
init_offset: <float>
init_cutoff: <float>
init_epsilon: <float>
init_exponent: <int>
init_time: <float>
init_velocity: <float>
final_time: <float>
final_velocity: <float>
times: [<float>, ...]
velocities: [<float>, ...]
velocity_profile: <step_or_linear>
freeze_at_end: <bool>
csv_filename: <string>
csv_separator: <string>
csv_append: <bool>
Parameters
init_normal: Vec3d, default [1,0,0] # Unit normal of the plane.
init_offset: float, default 0.0 # Initial signed offset.
init_cutoff: float, required # Interaction range.
init_epsilon: float, required # Energy scale.
init_exponent: int, default 12 # Power-law exponent.
init_time: float, see note # Time at which the wall starts moving.
init_velocity: float, see note # Wall speed along normal, from init_time.
final_time: float, optional # Time at which the ramp to final_velocity completes; also when motion freezes and epsilon is forced to 0.
final_velocity: float, optional # Speed after final_time (constant acceleration between init_time and final_time); defaults to init_velocity. Requires final_time.
times: list of floats, see note # Waypoint times for multi-shock (reshock) loading — alternative to init_time/final_time/final_velocity.
velocities: list of floats, see note # Wall speed at each waypoint in times — same length as times.
velocity_profile: string, default "step" # "step": velocity jumps to each waypoint's value and holds. "linear": velocity ramps continuously between waypoints.
freeze_at_end: bool, default true # true: motion and epsilon freeze once the last waypoint/final_time is reached. false: wall keeps moving at the last velocity forever.
csv_filename: string, optional # If set, rank 0 appends one row per call (time, position, velocity, acceleration) to this file.
csv_separator: string, default "," # Field separator used in the CSV output.
csv_append: bool, default false # Append to an existing csv_filename instead of truncating it.
Two mutually exclusive ways to drive the wall: init_time+init_velocity (+ optional final_time/final_velocity) for a single ramp, or times+velocities (at least 2 strictly-increasing waypoints) for multi-shock/reshock loading — the two forms cannot be combined. Before motion starts, offset stays at init_offset; once frozen (final_time reached, or the last waypoint reached with freeze_at_end: true), epsilon is forced to 0, so the wall it feeds is effectively removed.
Usage example
left_wall:
- move_plane_wall:
init_normal: [1.0, 0.0, 0.0]
init_offset: 0.0
init_cutoff: 5.0 ang
init_epsilon: 1.0e-19 J
init_time: 10.0 ps
init_velocity: 0.01 ang/ps
final_time: 50.0 ps
final_velocity: 0.05 ang/ps
csv_filename: wall_trajectory.csv
- plane_wall
+compute_force: [ left_wall ]
Usage example
# multi-shock (reshock) loading
left_wall:
- move_plane_wall:
init_offset: 0.0
init_cutoff: 5.0 ang
init_epsilon: 1.0e-19 J
times: [10.0 ps, 30.0 ps, 50.0 ps, 80.0 ps]
velocities: [0.01 ang/ps, 0.03 ang/ps, 0.06 ang/ps, 0.0 ang/ps]
velocity_profile: step
- plane_wall
+compute_force: [ left_wall ]
Cylindrical Wall¶
cylinder_wall¶
Syntax
cylinder_wall:
origin: <Vec3d>
axis: <Vec3d>
radius: <float>
cutoff: <float>
exponent: <int>
epsilon: <float>
Parameters
origin: Vec3d, default [0,0,0] # Any point on the cylinder's axis.
axis: Vec3d, default [0,0,1] # Cylinder axis direction — must be a unit vector.
radius: float, required # Cylinder radius.
cutoff: float, required # Interaction range from the cylinder surface.
exponent: int, default 12 # Power-law exponent.
epsilon: float, default 1.0e-19 J # Energy scale.
The cylinder is infinite along axis (unbounded ends). Typically fed by move_cylinder_wall rather than set directly.
Usage example
confinement_wall:
- cylinder_wall: { origin: [0.0,26.4,26.4] ang, axis: [1.0,0.0,0.0], radius: 20.0 ang, cutoff: 2.2 ang, epsilon: 1.0e-19 J }
+compute_force: [ confinement_wall ]
move_cylinder_wall¶
Syntax
move_cylinder_wall:
init_origin: <Vec3d>
init_axis: <Vec3d>
init_radius: <float>
init_cutoff: <float>
init_epsilon: <float>
init_exponent: <int>
init_time: <float>
init_velocity: <float>
direction: <Vec3d>
init_translation_velocity: <float>
final_time: <float>
final_velocity: <float>
final_translation_velocity: <float>
csv_filename: <string>
csv_separator: <string>
csv_append: <bool>
Parameters
init_origin: Vec3d, default [0,0,0] # Initial point on the cylinder's axis.
init_axis: Vec3d, default [0,0,1] # Cylinder axis direction — passed through unchanged, it only orients the cylinder and never moves.
init_radius: float, required # Initial radius.
init_cutoff: float, required # Interaction range.
init_epsilon: float, required # Energy scale.
init_exponent: int, default 12 # Power-law exponent.
init_time: float, required # Time at which the wall starts changing.
init_velocity: float, default 0.0 # Radius growth rate, from init_time (negative = converging/imploding cylinder).
direction: Vec3d, default [0,0,0] # Unit vector for optional origin translation, independent of axis.
init_translation_velocity: float, default 0.0 # Translation speed of origin along direction.
final_time: float, optional # Time the ramp(s) to the final_* values complete; also when motion freezes and epsilon is forced to 0.
final_velocity: float, optional # Radius growth rate after final_time; defaults to init_velocity. Requires final_time.
final_translation_velocity: float, optional # Translation speed after final_time; defaults to init_translation_velocity. Requires final_time.
csv_filename: string, optional # If set, rank 0 appends one row per call (time, radius, radius_velocity, radius_acceleration, origin_x/y/z, translation_velocity, translation_acceleration) to this file.
csv_separator: string, default "," # Field separator used in the CSV output.
csv_append: bool, default false # Append to an existing csv_filename instead of truncating it.
Radius and origin position evolve independently — radius change is along the cylinder's own growth (init_velocity), and translation is along an unrelated direction, useful for an indentor moving through the sample perpendicular to the confinement axis. init_velocity defaults to 0, so this operator can be used for translation-only motion.
Usage example
confinement_wall:
- move_cylinder_wall:
init_origin: [0.0,66.0,135.0] ang
init_axis: [1.0,0.0,0.0]
init_radius: 50.0 ang
init_cutoff: 2.2 ang
init_epsilon: 1.0e-19 J
init_time: 0.5 ps
direction: [0.0,0.0,1.0]
init_translation_velocity: -200 m/s
csv_filename: cylinder_wall_trajectory.csv
- cylinder_wall
+compute_force: [ confinement_wall ]
Spherical Wall¶
sphere_wall¶
Parameters
center: Vec3d, default [0,0,0] # Sphere center.
radius: float, required # Sphere radius.
cutoff: float, required # Interaction range from the sphere surface.
exponent: int, default 12 # Power-law exponent.
epsilon: float, default 1.0e-19 J # Energy scale.
Typically fed by move_sphere_wall rather than set directly.
Usage example
confinement_wall:
- sphere_wall: { center: [50.0,50.0,50.0] ang, radius: 45.0 ang, cutoff: 2.2 ang, epsilon: 1.0e-19 J }
+compute_force: [ confinement_wall ]
move_sphere_wall¶
Syntax
move_sphere_wall:
init_center: <Vec3d>
init_radius: <float>
init_cutoff: <float>
init_epsilon: <float>
init_exponent: <int>
init_time: <float>
init_velocity: <float>
direction: <Vec3d>
init_translation_velocity: <float>
final_time: <float>
final_velocity: <float>
final_translation_velocity: <float>
csv_filename: <string>
csv_separator: <string>
csv_append: <bool>
Parameters
init_center: Vec3d, default [0,0,0] # Initial sphere center.
init_radius: float, required # Initial radius.
init_cutoff: float, required # Interaction range.
init_epsilon: float, required # Energy scale.
init_exponent: int, default 12 # Power-law exponent.
init_time: float, required # Time at which the wall starts changing.
init_velocity: float, default 0.0 # Radius growth rate, from init_time (negative = converging/imploding sphere).
direction: Vec3d, default [0,0,0] # Unit vector for optional center translation.
init_translation_velocity: float, default 0.0 # Translation speed of center along direction.
final_time: float, optional # Time the ramp(s) to the final_* values complete; also when motion freezes and epsilon is forced to 0.
final_velocity: float, optional # Radius growth rate after final_time; defaults to init_velocity. Requires final_time.
final_translation_velocity: float, optional # Translation speed after final_time; defaults to init_translation_velocity. Requires final_time.
csv_filename: string, optional # If set, rank 0 appends one row per call (time, radius, radius_velocity, radius_acceleration, center_x/y/z, translation_velocity, translation_acceleration) to this file.
csv_separator: string, default "," # Field separator used in the CSV output.
csv_append: bool, default false # Append to an existing csv_filename instead of truncating it.
Same shape as move_cylinder_wall above, minus axis — a sphere has no orientation, so radius growth and center translation are the only two independent motions.