# Spectral series' formula of a given atom (other than hydrogen-like)?

The hydrogen spectral series is given by the Rydberg formula:

The energy differences between levels in the Bohr model, and hence the wavelengths of emitted/absorbed photons, is given by the Rydberg formula:

$${1 \over \lambda} = R \left( {1 \over (n^\prime)^2} - {1 \over n^2} \right) \qquad \left( R = 1.097373 \times 10^7 \ \mathrm{m}^{-1} \right)$$ where $n$ is the upper energy level, $n'$ is the lower energy level, and $R$ is the Rydberg constant.

There is a similar formula for every hydrogen-like atom.

Question: Is there a theoretic formula for the spectral series of a given atom (other than hydrogen-like)? Else, why, what are the difficulties?