public HashMap<String, Double> calculateIDFOnly() {
		IndexReader reader = null;
		HashMap<String, Double> inverseDFMap = new HashMap<>();
		try {
			reader = DirectoryReader.open(FSDirectory
					.open(new File(indexFolder).toPath()));
			// String targetTerm = "breakpoint";

			Fields fields = MultiFields.getFields(reader);
			for (String field : fields) {
				Terms terms = fields.terms(field);
				TermsEnum termsEnum = terms.iterator();
				BytesRef bytesRef;
				while ((bytesRef = termsEnum.next()) != null) {
					if (termsEnum.seekExact(bytesRef)) {
						String term = bytesRef.utf8ToString();
						this.keys.add(term);
					}
				}
			}
			// now go for the IDF
			int N = reader.numDocs();
			double maxIDF = 0;
			for (String term : this.keys) {
				Term t = new Term(FIELD_CONTENTS, term);
				int docFreq = reader.docFreq(t);
				double idf = getIDF(N, docFreq);
				if (!inverseDFMap.containsKey(term)) {
					inverseDFMap.put(term, idf);
					if (idf > maxIDF) {
						maxIDF = idf;
					}
				}
			}
			// now normalize the IDF scores
			for (String term : this.keys) {
				double idf = inverseDFMap.get(term);
				idf = idf / maxIDF;
				inverseDFMap.put(term, idf);
			}

		} catch (Exception exc) {
			// handle the exception
		}
		return inverseDFMap;
	}