The concept of the molecular orbital composition is often involved in quantum chemistry literatures. However, no enough emphasis has been placed on the corresponding calculation methods, and even there exist some serious misunderstandings. In this article, the basic concepts and calculation methods of composition of basis functions, atomic orbitals and atoms in molecular orbitals are discussed in detail, the differences between various methods are analyzed and compared with examples, meanwhile, the problems in calculations and analyses that need to be noticed are pointed out, the suggestions for choosing appropriate calculation methods are also given.

Key words: orbital composition, basis function, atomic orbital, atomic orbitals in molecular environments, natural atomic orbital