The Variational Method
The variational method is the other main approximate method used in quantum
mechanics. Compared to perturbation theory, the variational method can be more
robust in situations where it's hard to determine a good unperturbed Hamiltonian (i.e.,
one which makes the perturbation small but is still solvable). On the other hand, in
cases where there is a good unperturbed Hamiltonian, perturbation theory can be more
efficient than the variational method.
The basic idea of the variational method is to guess a ``trial'' wavefunction for the
problem, which consists of some adjustable parameters called ``variational
parameters.'' These parameters are adjusted until the energy of the trial wavefunction
is minimized. The resulting trial wavefunction and its corresponding energy are
variational method approximations to the exact wavefunction and energy.
Why would it make sense that the best approximate trial wavefunction is the one with
the lowest energy? This results from the Variational Theorem, which states that the
energy of any trial wavefunction
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 Fall '10
 LauraChoudry
 Chemistry, variational method, trial wavefunction

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