where, for the N -electron system, Ĥ is the Hamiltonian, E is the total energy, T ^ ^ is the kinetic energy, V ^ ^ is the potential energy from the external field due to positively charged nuclei, and Û is the electron–electron interaction energy. The operators T ^ ^ and Û are called universal operators, as they are the same for any N -electron system, while V ^ ^ is system-dependent. This complicated many-particle equation is not separable into simpler single-particle equations because of the interaction term Û.