Van Vleck’s theorem

Van Vleck’s theorem states that the magnetic properties of a quantum system, such as an atom with electrons, can be predicted by analyzing how the system's electronic states respond to magnetic fields. Specifically, it connects the observable magnetic susceptibility to the averages of the electrons' magnetic moments within their energy states. Essentially, it provides a way to calculate how a material reacts to magnetic fields based on the quantum behavior of its electrons, helping scientists understand and predict magnetic phenomena in complex systems.