libint is two things:
Developers of scientific software, such as for molecular electronic structure. In molecular electronic structure theory Gaussian basis sets are standard because they allow efficient evaluation of matrix elements of operators (molecular integrals). Modern electronic structure programs spend considerable portion of their runtime computing the Coulomb two-electron integrals. While anyone can compute Gaussian integrals using simple formulas (see, for example, here and here), the efficient evaluation of many-body can be (relatively) complicated. Libint is an open library that anyone can use to compute a variety of two-electron integrals, most importantly the Coulomb two-electron integrals and their arbitrary-order geometric derivatives, over Gaussians of arbitrary angular momentum. Among other notable features is the support for the nonstandard two-electron integrals that appear in explicitly correlated R12 methods.
The latest major version of Libint is 2 . Libint2 has been in use internally since 2004; in fact it is more stable than version 1. It has now been improved to production-ready status and officially released on April 25, 2011.
The vast majority of users will not be interested in tinkering with the compiler -- and, thus, are recommended to download one of pre-generated libraries:
If you want to download the Libint compiler to generate a custom Libint library -- you can check it out from the source repository! (Read the wiki for instructions).
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
Refer to the full text of the GNU General Public License. You can obtain the most up-to-date version of GPL if you write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
For version 2: Copyright (C) 2004-2014 Edward F. Valeev.