These instances were used in Track B of The Second Parameterized Algorithms and Computational Experiments Challenge (PACE 2017). The aim of this track was to compute a minimum-fill-in of the given graph. A fill-in is a set of edges whose insertion makes the input graph chordal. A chordal graph is a graph that contains no cycle of length at least four as induced subgraph. Half of the instances were made public before the challenge and the other half remained hidden until the conclusion of the challenge. Details can be found in the official PACE 2017 report.
Each .graph-file contains a list of vertex-pairs, constituting an undirected edge. Each line corresponds to one edge. The vertices of the edge are separated by a whitespace. Vertex ids are integers of value at least 1. Lines starting with '#' are comments.