The potential functions over these cliques are defined as 7where the feature is defined as the log probability of generating an expansion bigram given Unlike the language models used for document ranking (e.g., Zhai and Lafferty 2001), we cannot compute the bigram probability by simply counting the relative frequency of in because the query is usually very short and the bigram is unlikely to occur.
we approximate the bigram probability by assuming that the words in are independent with each other.