ABSTRACT
The virial theorem in quantum mechanics is a full analogue of this theorem in classical mechanics. An important consequence of the variational principle is the so called Hellmann-Feynman theorem. Molecules, as distinct from atoms, contain more than one atomic nucleus. Any molecular process somehow involves a nuclear rearrangement. Moreover even in the molecular ground state the nuclei are in motion and execute small-amplitude oscillations around their equilibrium positions (zero-point oscillations). The molecular problem would be simplified if we could treat separately the nuclei and electrons, that is, separate the nuclear and electronic coordinates. The bar in the parentheses separates the active electronic coordinates from the nuclear coordinates that serve as parameters of the electronic Hamiltonian. Classically speaking, a stationary point corresponds to a stable or unstable equilibrium of the system. Local minima on the surface are stable equilibrium points of the system.
