Even a single atom can be very complicated. Uranium has 92 electrons, each interacting with the nucleus and all 91 other electrons.

The wavefunction of even a single electron is really an infinite set of numbers, one for every position in space.

Approximations are important: Assume each electron interacts only with the average of all the others. Assume that the wavefunction is simple enough to be described by a large but finite set of numbers.

With better approximation methods and better computers, our ability to predict chemical and physical properties now reaches to systems of several hundred molecules.

Predictions still have to be tested, however, and the approximations are often wrong.

Examples --- What this is an example of

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