Below is the original Readme from the sundials/README file.
The orginal sundials package was obtained from
http://www.llnl.gov/CASC/sundials/
For purposes of incorporating cvodes and ida into NEURON I have
simplified the directory structure by combining source and include
files into the cvodes, and ida directories and combined
nvec_ser, nvec_par, shared/include, and shared/src *.[ch] files
into the shared subdirectory, eliminated
the doc, kinsol, cvode directories and other subdirectories, and
replaced the
Makefile.in files with Makefile.am files.
The reason that the sensitivity version of cvode is used is that the
previous version of cvode did not
support the tstop functionality needed by at_time.

Note that the version 2.0 now has tstop functionality for cvode but
due to inertia we still use cvodes. Also version 2.0 incorporates
autoconf (although configure.in is not distributed in the sundials.tar.gz
file) so we make use of config.hin. It is conceivable that the
sundials.tar.gz with all its subdirectory structure along with configure
could be used now. But for least porting, cvs, Makefile.am
and distribution difficulty we retain the previous subdirectory structure.
-----------------------------------------------------------------------------

                            SUNDIALS 
    SUite of Nonlinear and DIfferential/ALgebraic equation Solvers
                   Release 2.0.1, January 2005
     Peter Brown, Aaron Collier, Keith Grant, Alan Hindmarsh, 
      Steve Lee, Radu Serban, Dan Shumaker, Carol Woodward
          Center for Applied Scientific Computing, LLNL


The family of solvers referred to as SUNDIALS consists of solvers CVODE 
(for ODE systems), CVODES (ODE with sensitivity analysis capabilities),
IDA (for differential-algebraic systems), and KINSOL (for nonlinear 
algebraic systems), 

The various solvers of this family share many subordinate modules.
For this reason, it is organized as a family, with a directory structure 
that exploits that sharing. Each individual solver includes documentation 
on installation, along with full usage documentation.

The following is a list of the solver packages presently available.

CVODE:  A solver for stiff and nonstiff ODE systems y' = f(t,y),
        with serial and parallel versions.

CVODES: A solver for stiff and non-stiff ODE systems with sensitivity
        analysis capabilities, with serial and parallel versions.

IDA:    A solver for differential-algebraic systems F(t,y,y') = 0, with
        serial and parallel versions.

KINSOL: A solver for nonlinear algebraic systems F(u) = 0, with serial
        and parallel versions.

Warning to users who receive more than one of these individual solvers
at different times: The mixing of old and new versions SUNDIALS may fail.  
To avoid such failures, obtain all desired solvers at the same time.

For installation directions see the file INSTALL_NOTES.

For additional information on a particular solver, see the README file in
the solver directory.

                        Release history

+----------+------------------------------------------------------+
|          | SUNDIALS |            Solver version                 |
|   Date   |          +----------+----------+----------+----------+
|          | release  |   CVODE  | CVODES   |   IDA    |  KINSOL  |
+----------+----------+----------+----------+----------+----------+
| Jul 2002 |   1.0    |    2.0   |    1.0   |    2.0   |    2.0   |
| Dec 2004 |   2.0    |  2.2.0   |  2.1.0   |  2.2.0   |  2.2.0   |
| Jan 2005 |   2.0.1  |  2.2.1   |  2.1.1   |  2.2.1   |  2.2.1   |
+----------+----------+----------+----------+----------+----------+