Molecular orbitals form from "allowed" interactions between atomic orbitals. (The interactions are "allowed" if the symmetries (determined from group theory) of the atomic orbitals are compatible with each other.) Chemists study atomic orbital interactions. These interactions come from the overlap (a measure of how well two orbitals constructively interact with one another) between two atomic orbitals. The overlap is important if the atomic orbitals are close in energy. Finally, the number of MOs in a molecule must equal the number of atomic orbitals in the atoms being brought together to form the molecule.