ERROR opening https://cactus.nci.nih.gov/chemical/structure/InChI=1/C4H6/c1-2-4-3-1/h1-2H,3-4H/file?format=sdf&get3d=True  -- InChI=1/C4H6/c1-2-4-3-1/h1-2H,3-4H could not be loaded.