Having negative perplexity apparently is due to infinitesimal probabilities being converted to the log scale automatically by Gensim, but even though a lower perplexity is desired, the lower bound value denotes deterioration (according to this), so the lower bound value of perplexity is deteriorating with a larger ...

Should perplexity be higher or lower?

A lower perplexity score indicates better generalization performance. In essense, since perplexity is equivalent to the inverse of the geometric mean, a lower perplexity implies data is more likely. As such, as the number of topics increase, the perplexity of the model should decrease.

Is low perplexity good?

In information theory, perplexity is a measurement of how well a probability distribution or probability model predicts a sample. ... A low perplexity indicates the probability distribution is good at predicting the sample.