add_spring

This keyword is used to add a spring between a ghost atom and the centroid of atoms in a selected group.

Syntax

This keyword is used in one of the following two ways:

add_force <group_method> <group_id> <ghost_vx> <ghost_vy> <ghost_vz> couple <k_couple> <R0> <offset_x> <offset_y> <offset_z>  # usage 1
add_force <group_method> <group_id> <ghost_vx> <ghost_vy> <ghost_vz> decouple <k_decouple_x> <k_decouple_y> <k_decouple_z> <offset_x> <offset_y> <offset_z>  # usage 2

The function of the coupled spring potential is given by the equation:

\[U_{\mathrm{couple}} = \frac{1}{2} k_{\mathrm{couple}} (R - R_0)^2\]

where \(R\) is the distance between the ghost atom and the centroid of the selected group. The function of the decoupled spring potential is given by the equation:

\[U_{\mathrm{decouple}} = \frac{1}{2} k_{\mathrm{decouple},x} (x - x_{\mathrm{ghost}})^2 + \frac{1}{2} k_{\mathrm{decouple},y} (y - y_{\mathrm{ghost}})^2 + \frac{1}{2} k_{\mathrm{decouple},z} (z - z_{\mathrm{ghost}})^2\]

where \((x_{\mathrm{ghost}}, y_{\mathrm{ghost}}, z_{\mathrm{ghost}})\) is the position of the ghost atom and \((x, y, z)\) is the position of the centroid of the selected group.

  • Force is added to atoms in group group_id of group method group_method.

  • In the first usage, a spring is coupled to the ghost atom with spring constant k_couple and equilibrium distance R0.

  • In the second usage, a spring is decoupled from the ghost atom with spring constants k_decouple_x, k_decouple_y, and k_decouple_z in the x, y, and z directions, respectively.

  • The ghost atom starts at the position of the centroid of the selected group plus the offset vector (offset_x, offset_y, offset_z), and moves with velocity (ghost_vx, ghost_vy, ghost_vz).

  • Force is in units of eV/Å, distance is in units of Å, velocity is in units of Å/step, and spring constant is in units of eV/Ų.

Example 1

Add a coupled spring with spring constant 10 eV/Ų and equilibrium distance 0 Šbetween a static ghost atom and the centroid of atoms in group 2 of group method 0. The ghost atom is initially in the same position as the centroid:

add_spring    ghost_com 0 2 0 0 0 couple 10 0 0 0 0

Example 2

Add a decoupled spring with spring constants 10 eV/Ų in the x direction and 0 eV/Ų in the y and z directions between a ghost atom moving with velocity (0.00005, 0, 0) Å/step and the centroid of atoms in group 2 of group method 0. The ghost atom is initially in the same position as the centroid:

add_spring    ghost_com 0 2 0.00005 0 0 decouple 10 0 0 0 0 0

Note

This keyword can be used multiple times during a run.