Based on the OPP technique and the HELAC framework, HELAC-1LOOP is a program
that is capable of numerically evaluating QCD virtual corrections to scattering
amplitudes. A detailed presentation of the algorithm is given, along with
instructions to run the code and benchmark results. The program is part of the
HELAC-NLO framework that allows for a complete evaluation of QCD NLO
corrections.Comment: minor text revisions, version to appear in Comput.Phys.Commu
