[Nauty] very hard isomorphism test cases

[jrainy at sympatico.ca]

Hi,   
   
I'm not sure this is the right place to ask, but it seems   
like a good try. I'm looking for tough isomorphism test   
cases. Ideally, what I'd like to find is a set of pair of   
(undirected non-reflexive) graphs labelled isomorphic or  
not. The isomorphism need not be shown.   
  
I could easily generate test cases, but I'd like to be  
certain that the isomorphism are tough ones. For example,  
the toughest problem that Nauty ever solved would be a good  
test case. ( not that I hope to solve it, but for testing )  
 
Any pointers, links, and/or graphs in any format would be 
greatly appreciated. I'm not interested in sub-graph 
isomorphism, though. 
 
Thanks in advance.  
  
Jeffrey.