Hi this question is mainly for Stephen, I was working on using the ellipsoid branch of YT to do some analysis on the 3200 cube haloes found. I came across the same problem you described earlier where calculating for tB_index I get nan sometimes, but this is happening even after your fix. I dug through and found out that this happens because pHOP found some halo that have only 1 particle. This becomes problematic with my geometric ellipsoid, it requires at least 4 particles distributed in different planes. When the ellipsoid method is trying to find a particle to satisfy the equation of an ellipse in 2D, it will go through the list (of 1 element in this case), and find that it returns nan for the length it calculates. So even though the function nanargmax() is used to pick out biggest value of non-nan element index, with a list of only 1 nan, it still gets selected and the method fails. Currently my temporary solution is to tell the function that, if it encounters a halo with less than 4 particles, to returns all 0s for the ellipsoid attribute (A,B,C,e1x,y,z,tilt). I was under the impression from our previous talk that there's a lowest number threshold that pHOP requires to form a halo, I thought the number was 10 particles at least. Am I just remembering things wrong? It has been a couple of years... From G.S.