  New \textit{ab initio} calculations of the isotopic reactor antineutrino
fluxes are provided with exact numerical calculations of the lepton wave
functions, assuming all the decay branches are allowed GT transitions. We
illustrate that the analytical Fermi function and finite size effect each could
have the largest spectral deviation of $\mathcal{O}(10\%)$, whereas the effect
of their combination could result in spectral deviations at the level of
5%-10%. Meanwhile, we also find that several forms of the extended charge
distributions have negligible effects on the spectral variation. Using the
state-of-the-art nuclear databases, compared to usual \textit{ab initio}
calculations using the analytical single beta decay spectrum, our new
calculation shows sizable but opposite spectral deviations at the level of
2%-4% for the cumulative antineutrino and electron energy spectra which may
partially contribute to the observed spectral excess in the high energy
antineutrino range. Finally we observe that the {bias} of analytical beta decay
spectrum approximation is rather universal for all the four fissionable
isotopes.
