In the SMT community, researchers have developed standard, proven alignment tools such as GIZA++ (Och and Ney, 2003), which can be used to train IBM Models 1-5.
there is one fundamental problem with the IBM models (Brown et al., 1993): each word on one side can be traced back to exactly one particular on the other word (or the null token which indicates the word aligns to no word on the other side).