Use x-axis along the particle's quaternion.

For mode = velocity, the active force acts along the velocity vector of each atom. This can

be interpreted as a velocity-dependent friction, such as proposed by "(Erdmann)"_#Erdmann.

For mode = quaternion the force is along the axis obtained by rotating the z-axis along the

atom's quaternion. In other words, the force is along the z-axis in the atom's body

For mode = quaternion the force is along the axis obtained by rotating the x-axis along the

atom's quaternion. In other words, the force is along the x-axis in the atom's body

frame. This mode requires all atoms in the group to have a quaternion, so atom_style should

either be ellipsoid or body.  In combination with Langevin thermostat for translation and

rotation in the overdamped regime, the quaternion mode corresponds to the active Brownian

        continue;

      }

      double f_act[3] = { 0.0, 0.0, 1.0 };

      double f_act[3] = { 1.0, 0.0, 0.0 };

      double f_rot[3];

      double *quat  = bonus[ellipsoid[i]].quat;