public Point3i getBondCenterScreen(int index) {
		if (index < 0 || index >= modelManager.frame.bondCount)
			return null;
		Bond b = modelManager.frame.bonds[index];
		return new Point3i((b.atom1.screenX + b.atom2.screenX) >> 1, (b.atom1.screenY + b.atom2.screenY) >> 1,
				(b.atom1.screenZ + b.atom2.screenZ) >> 1);
	}