_Avoided crossing_ is the phenomena that a pair of energy levels connected by a [perturbation](time-dependent%20quantum%20perturbation%20theory.md) never cross. The mathematical result that gives rise to this is referred to as the _no-level-crossing theorem._ %%This concept is introduced on pg 310 of Sakurai's book. Sakurai does not elaborate on it very thoroughly so more sources are needed. This is of course a very important concept in a lot of physics however.%% #QuantumMechanics/StationaryStateQuantumSystems #QuantumMechanics/ApproximationMethods/QuantumPerturbationTheory