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								April 20 1993

    This is a Fortran-77 version of the tree code.  The algorithm used to
compute gravitational forces has asymptotic complexity of O(N log N).  This
is achieved by building a tree structure to represent the mass distribution
and computing forces by a partial recursive descent of this tree.  See "A
Hierarchical O(N log N) Force Calculation Algorithm", by Joshua Barnes and
Piet Hut, Nature, 324, 446, and "Error Analysis of a Tree Code", by Joshua
Barnes and Piet Hut, Astrophysical Journal Supplement, 70, 389, for more
information.

    This version is based on a Fortran implementation kindly supplied by
Lars Hernquist.  It includes optional quadrupole corrections for body-cell
interactions, much improving the accuracy of the method; see "Performance
Characteristics of Tree Codes", by Lars Hernquist, Astrophysical Journal
Supplement, 64, 715.  Several additional refinements were taken from a
similar code developed by Jun Makino.

    The source files for the tree code have been bundled together for ease
of distribution.  On a UNIX system, you can split this file simply by
invoking the Bourne shell on it (use the sh command).  On other systems, you
may have to edit this file and split it up by hand; note that file X is
delimited by lines containing the message End of X.  Once you have split
this file, you can compile it by running make on a UNIX system.  On other
systems, you will have to figure out how to run the compiler; take a look at
the makefile for hints.

    We offer NO WARRANTY on the software supplied here -- use it at your own
risk.  We impose NO RESTRICTIONS on use and distribution of our code -- you
are free to use this software for any application, modify it in any way, use
pieces in other programs, and give it away to anyone else.  We encourage you
to extend these rights to others who receive this software from you (in this
we follow the philosophy and intentions of the Free Software Foundation).
We would appreciate a brief acknowledgement in any scientific papers you
write using our code, and we would also like to receive preprints of such
papers.

    Have fun and stay in touch.

	Joshua E. Barnes

	Institute for Astronomy
	University of Hawaii
	2680 Woodlawn Drive
	Honolulu, Hawaii  96822

	(808) 956-8138

	barnes@zeno.ifa.hawaii.edu


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