The operator 𝐻 ̂ contains all the terms describing the interactions between the particles in a chemical system. Except for very specific environments, these are nuclei and electrons. Thus, 𝐻 ̂ contains the kinetic energy of nuclei (𝑇 ̂n) and electrons (𝑇 ̂e), and potential energy terms of the interactions between nuclei (𝑉 ̂nn ), electrons (𝑉 ̂ee ) and between electrons and nuclei (𝑉 ̂ne ). In most computations, the Schrödinger equation for the electrons can be solved for a stationary arrangement of nuclei since the latter are much heavier and move thus much slower. This so-called Born-Oppenheimer approximation leads to 𝑇 ̂n = 0 and 𝑉 ̂nn being constant and the following 𝐻 ̂ is used: