Idf values of English words I'm working on keyword/phrase extraction from a single document. I started by doing term frequency analysis, but this returns words like "new" which aren't very helpful. So I want to penalize the common words and phrases, for which we normally use idf (inverse document frequency). But since it's for a single document, I'm not sure how to do idf analysis. 

Is it possible to use tf-idf method with pre-calculated idf values for (all?) words?
And are such values available somewhere?
I don't believe that there are any precalculated idf values out there. Inverse Document Frequency (idf) is the inverse of the number of documents in which a particular word appears in your corpus. If you only have one document, I'm afraid that value is simply 1. 

However, if you are looking to get rid of words such as the, as, and it which don't carry much meaning, nltk in Python has some useful tools to remove these "stop words" from your document and might help you. 

Here is a helpful example.