Given the challenges though, as illustrated by the examples above, we are still optimistic that the benefit and novel insight of applying density-based ideas to appreciate traditional chemical concepts are enormous and conspicuous. This indicates that much understanding can be achieved without invoking the picture of orbitals. In other words, molecular orbitals are not mandatory per se to explain and rationalize our experimental findings. This does not mean, however, that density-based and orbital-based descriptions and understandings are mutually exclusive. Indeed, resulted from alternative solutions of the same Schrödinger equation, these two pictures are and should be complementary to each other.