Two-Loop Matching Coefficients for Heavy Quark Currents
  In this paper we consider the matching coefficients up to two loops between Quantum Chromodynamics (QCD) and Non-Relativistic QCD (NRQCD) for the vector, axial-vector, scalar and pseudo-scalar currents. The structure of the effective theory is discussed and analytical results are presented. Particular emphasis is put on the singlet diagrams. 
B.A. Kniehl, A. Onishchenko, J.H. Piclum, M. Steinhauser