MultiDark Planck 2 Catalogs Public Release, version 1.0 (Nov. 15, 2015).

Each "tree" file contains complete merger histories for halos in a (100 Mpc/h)^3
subvolume of MDPL2.  There are 1000 files in total, labeled as tree_X_Y_Z.dat, all in
ASCII format.

Each tree file begins with an explanatory header followed by a number on a single line,
which refers to the total number of merger trees contained in the file.  Every halo and
subhalo at z=0 will have a distinct merger tree; trees are delineated within the file
with "#tree XYZ" tags, where XYZ is the ID of the z=0 halo.

Halos in a merger tree are stored with one line per halo.  The are presented in breadth-first
order, meaning that all the halos in the merger tree at one scale factor are grouped together.
Alternate indices (i.e., the depth-first order index) are also included to facilitate alternate
tree-walking approaches.  The details of what is in each column is contained in the header of
the tree; this includes halo positions, velocities, masses, and many other useful properties.

More complete column descriptions can be found in: 
http://arxiv.org/abs/1602.04813 (Rodriguez-Puebla et al. 2016).

There are also two other useful files.  "forests.list" contains a list of z=0 halo ids and the
corresponding forest ID.  If two halos share the same forest ID, then their merger trees
have cross-references to each other (as may happen for subhalos at z=0 or for flyby halos).
Loading in all trees in a forest guarantees that there are no cross-references outside the
forest.  The second useful file, "locations.dat", contains a list of z=0 halo ids and the
corresponding filename and location where the tree data is stored.  Because z=0 halo ids
are contiguous, it is straightforward to use a plain array structure to store the contents
of both files.

If you use these tree files, please cite the following three papers:

The original simulation paper: https://arxiv.org/abs/1411.4001 (Klypin et al. 2016)
The Rockstar halo finder paper: https://arxiv.org/abs/1110.4372 (Behroozi, Wechsler, and Wu, 2013).
The consistent merger trees paper: https://arxiv.org/abs/1110.4370 (Behroozi et al. 2013).

Please also add the following acknowledgements:

The MDPL2 simulation and halo finding has been performed at LRZ Munich within the project pr87yi.  The CosmoSim database (www.cosmosim.org) providing the file access is a service by the Leibniz-Institute for Astrophysics Potsdam (AIP).