  The purpose of this paper is to calculate the two-photon decay rate
corresponding to the two-photon transitions nS->1S and nD->1S in hydrogenlike
ions with a low nuclear charge number Z (for principal quantum numbers n =
2,...,8. Numerical results are obtained within a nonrelativistic framework, and
the results are found to scale approximately as (Z alpha)^6/n^3, where alpha is
the fine-structure constant. We also attempt to clarify a number of subtle
issues regarding the treatment of the coherent, quasi-simultaneous emission of
the two photons as opposed to one-photon cascades. In particular, the gauge
invariance of the decay rate is shown explicitly.
