Using the single-electron wave function ψ(r) = N*exp( −ζr2 ) with ζ a variational parameter, how can we calculate the best approximation for the ground state energy of the hydrogen atom?
Variational principle can be used to calculate the upper bound of the ground state of a system. This is because
$$
\langle \psi | H | \psi \rangle \geq E_g
$$
where ##\psi## is an arbitrary normalized state and ##E_g## the system's ground state whose Hamiltonian is ##H##.