Most of the theoretical material used in MOPAC is given in the theory section. This section gives a description of the ideas, concepts, equations, and approximations used in MOPAC. A careful examination of these will provide enough information to allow a user to understand the methods used in MOPAC.

With only a few exceptions, the derivation of the theoretical concepts is not given because almost all the derivations are readily available in the literature. The only exceptions are for equations or concepts that are not well described in the literature, of these the most important is the overlap integral over atomic Slater orbitals. Some features of the output, a good example being State eigenvectors that involves the 3J symbol, are also described. On the whole, though, the theory section has been written with the intention of describing the theory used in MOPAC, and it is left to the reader to find out how the current state of the theoretical concepts came to be.