code: private void calcPrincipalRotationVector() {
  		Rotation rotation = rotationGroup.getRotation(0); // the rotation around the principal axis is the first rotation
  		AxisAngle4d axisAngle = rotation.getAxisAngle();
  		principalRotationVector = new Vector3d(axisAngle.x, axisAngle.y, axisAngle.z);
  	}
docs: Returns a vector along the principal rotation axis for the
  alignment of structures along the z-axis
  @return principal rotation vector