6. Approximations

• This section will teach us how to make approximations and assess closenss of approximation to reality in a systematic and organized fashion.

• We will learn two powerful frameworks for making aprpoximations in quantum mechanics: Variational methods and Pertrubation theory.

• Variational method will give us a stratihforward way of getting the be

• Pertrubation theory will give us simple yet incredibly powerful tool for gleaning inisghts into how pertrubing a hamitlonian affects the eigneufnctions and eigenvalues.

• Time-dependent perturbation theory will reveal where the origin of selection rules in spectroscopy.