* As an example, the following code creates a CustomExternalForce that attracts each particle to a target position (x0, y0, z0)
* As an example, the following code creates a CustomExternalForce that attracts each particle to a target position (x0, y0, z0)
* via a harmonic potential:
* via a harmonic potential:
*
*
* <tt>CustomExternalForce* force = new CustomExternalForce("k*((x-x0)^2+(y-y0)^2+(z-z0)^2");</tt>
* <tt>CustomExternalForce* force = new CustomExternalForce("k*((x-x0)^2+(y-y0)^2+(z-z0)^2)");</tt>
*
*
* This force depends on four parameters: the spring constant k and equilibrium coordinates x0, y0, and z0. The following code defines these parameters:
* This force depends on four parameters: the spring constant k and equilibrium coordinates x0, y0, and z0. The following code defines these parameters:
