Reducing full one-loop amplitudes to scalar integrals at the integrand   level
  We show how to extract the coefficients of the 4-, 3-, 2- and 1-point one-loop scalar integrals from the full one-loop amplitude of arbitrary scattering processes. In a similar fashion, also the rational terms can be derived. Basically no information on the analytical structure of the amplitude is required, making our method appealing for an efficient numerical implementation 
Giovanni Ossola, Costas G.Papadopoulos and Roberto Pittau