Molecular orbital theory, also known as molecular orbital theory or Mo method, was put forward by American chemist R.S.Mulliken and German physicist f. Hont hunt Hungary in 1932. This is one of the modern valence bond theories. The main points are as follows: the structure of molecules is discussed from the perspective of molecular integrity, and it is considered that after atoms form molecules, electrons no longer belong to a single atomic orbit, but to the molecular orbit of the whole molecule, and the molecular orbit is polycentric; Molecular orbitals are composed of atomic orbitals, which follow the principles of energy approximation, symmetry consistency (matching) and maximum overlap, that is, the so-called "triple bond principle". The principle of filling molecular orbitals with electrons in molecules also follows the lowest energy principle, Pauli exclusion principle and Hunter's law.

1927 put forward the famous valence bond theory. Subsequently, under the influence of Hont Hungary, Hungary, Malkern, Slade and John Leonard-Jones, molecular orbitals began to appear. So in the beginning, the molecular orbital theory was called Hont Hungary-Maliken theory. The concept of "orbit" was first put forward by Ma Liken in 1932.

By 1933, the molecular orbital theory has been widely accepted as an effective and useful theory. In fact, according to the description of German physical chemist Hooker, the first document using molecular orbital theory was published by Leonard Jones in 1929. The first quantitative calculation document using molecular orbital theory was published by Coulson in 1938, and the electronic wave function of hydrogen molecule was solved by self-consistent field theory.

At 1950, the molecular orbital is completely defined as the intrinsic function of the self-consistent field Hamiltonian, which indicates that the molecular orbital theory has developed into a rigorous scientific theory. Hartree-Fock method is a strict treatment method of molecular orbital theory. Although HF method was used to calculate the electronic structure of atoms at first, in molecular calculation, molecular orbits were developed according to a set of basis groups of atomic orbits, and Rotherham equation was developed in this way. Based on this, various ab initio quantum chemical calculation methods have been developed. At the same time, the molecular orbital theory has also been applied to a semi-empirical calculation with a more approximate method, which is called semi-empirical quantum chemical calculation method.

Molecular orbitals can be obtained by linear combination of atomic orbitals (LCAO). Several atomic orbitals can be combined into several molecular orbitals, some of which are formed by the superposition of two symmetrically matched atomic orbitals. The probability density of electrons between two atomic nuclei increases, and its energy is lower than that of the original atomic orbitals, which is beneficial to bonding, and is called bonding molecular orbitals, such as σ and π orbitals (axisymmetric orbitals); At the same time, these symmetrically matched two atomic orbitals will be subtracted to form another molecular orbital. In this way, the electron probability density between the two nuclei is very small, and its energy is higher than the original atomic orbit, which is not conducive to bonding. It is called anti-bond molecular orbitals, such as σ * and π * orbitals (mirror symmetry orbitals, and the sign of anti-bond orbitals is often added with "*" to distinguish bonding orbitals). Another special case is that the atomic orbitals that make up the molecular orbitals do not match in spatial symmetry, and the atomic orbitals do not overlap effectively. The energy of molecular orbitals obtained by combination is not obviously different from that of atomic orbitals before combination, and the obtained molecular orbitals are called non-bonding molecular orbitals.

The arrangement of electrons in molecular orbits also follows the same principle of atomic orbital electron configuration, namely Pauli exclusion principle, minimum energy principle and Hunder rule. In the specific arrangement, we must first know the order of energy levels of molecular orbitals. At present, this order is mainly determined by molecular spectroscopy experiments.

Only symmetrically matched atomic orbitals can be combined into molecular orbitals, which is called the symmetry matching principle. There are various types of atomic orbits, such as s, p, d, etc. From the geometric figures of their angular distribution functions, we can see that they have different spatial symmetry for some points, lines and surfaces. Whether the symmetry matches can be determined according to the symmetry of the positive and negative signs of the lobe in the angular distribution diagram of two atomic orbits relative to the bond axis (set as X axis) or the plane containing the bond axis.