What is a key concept in Variational Perturbation Theory?

Perturbative expansions are a central concept in Variational Perturbation Theory, providing a systematic way to approximate solutions to quantum mechanical problems. By expanding the solutions in terms of small parameters that represent the perturbations, we can calculate corrections to the unperturbed solutions. These expansions allow us to break down complex problems into simpler ones and obtain approximate solutions that are valid for small perturbations. Perturbative techniques are widely used in theoretical physics and quantum mechanics to study the behavior of systems under external influences or interactions.