  We present accurate time-dependent ab initio calculations on fully
differential and total integrated (generalized) cross sections for the
nonsequential two-photon double ionization of helium at photon energies from 40
to 54 eV. Our computational method is based on the solution of the
time-dependent Schroedinger equation and subsequent projection of the wave
function onto Coulomb waves. We compare our results with other recent
calculations and discuss the emerging similarities and differences. We
investigate the role of electronic correlation in the representation of the
two-electron continuum states, which are used to extract the ionization yields
from the fully correlated final wave function. In addition, we study the
influence of the pulse length and shape on the cross sections in time-dependent
calculations and address convergence issues.
