  The formalism that describes radiative-capture reactions at low energies
within an extended two-cluster potential model is presented. Construction of
the operator of single-photon emission is based on a generalisation of the
Siegert theorem with which the amplitude of the electromagnetic process is
constructed in an explicitly gauge-independent way. While the starting point
for this construction is a microscopic (single-nucleon) current model, the
resulting operator of low-energy photon emission by a two-cluster system is
expressed in terms of macroscopic quantities for the clusters and does not
depend directly on their intrinsic coordinates and momenta. The multichannel
algebraic scattering (MCAS) approach has been used to construct the initial-
and final-state wave functions. We present a general expression for the
scattering wave function obtained from the MCAS T matrix taking into account
inelastic channels and Coulomb distortion. The developed formalism has been
tested on the 3He(alpha,gamma)7Be reaction cross section at astrophysical
energies. The energy dependence of the evaluated cross section and S factor
agrees well with that extracted from measurement though the calculated
quantities slightly overestimate data.
