However, their localization can do the trick (see Fig. ), although the localization process may sound too artificial and begging the question for the taste of some physical chemistry purists, plus they lose any spectroscopical insight. In essence, each style of molecular orbital representation must be used for the purpose it was designed, for instance the orbitals localized in the bond can be used to define, quantify, and understand each bond (see for instance the NBO and Boys localization in Fig. ). quintuple, or up to a sextuple covalent bond; you might even have half a covalent bond (like in the 3-center-4 electron cases such as XeF2 or with F2 -), one and a half covalent bond (as in benzene), or another option if an odd number of electrons are involved. In this formal language, the covalent bond order is always a function of the number of electrons and the orbitals they occupy. However, this integer formality can disappear, producing fractional bond orders, for instance when there is superposition of localized orbitals, or when using multi-reference methods that result in fractional orbital occupancy. In other words, integer bond orders are only a formal idealization of real bonds. In contrast, when speaking about chemical bonds in its structural or energetic perspective we generally do not speak in terms of bond orders, but in terms of their quantitative strengths, just stating that a bond is weak or strong. But maybe the biggest disadvantage of the MO as a diagnostic of the chemical bond is, similar to the structural method, its difficulty to describe non-covalent bonds. MO theory as a computational approximation to the solution of Schrödinger equation can certainly compute the electrostatic and van der Waals bonding through ab initio or DFT methods, and there are orbital-based explanations to these interactions. Nevertheless, no significant density between atoms implies that there are no significant bonding orbitals between them, limiting the model. We cannot say that orbitals make the bonds when there are no orbitals into these bonds.