In quantum mechanics, perturbation theory's a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one. The idea's to start with a simple system for which a mathematical solution's known, and add an additional "perturbing" Hamiltonian representing a weak disturbance to the system. If the disturbance isn't too large, the various physical quantities associated with the perturbed system (for example its energy l… (More on Perturbative)