Finding Principal Null Direction for Numerical Relativists
  We present a new method for finding principal null directions (PNDs). Because our method assumes as input the intrinsic metric and extrinsic curvature of a spacelike hypersurface, it should be particularly useful to numerical relativists. We illustrate our method by finding the PNDs of the Kastor-Traschen spacetimes, which contain arbitrarily many $Q=M$ black holes in a de Sitter back-ground. 
Laurens Gunnarsen, Hisa-aki Shinkai and Kei-ichi Maeda