Libxc
=====

Libxc is a library of exchange-correlation functionals for
density-functional theory. The aim is to provide a portable, well
tested and reliable set of exchange and correlation functionals that
can be used by a variety of programs.

For more information, please check the manual at
http://www.tddft.org/programs/Libxc

INSTALLATION
============

Autotools
---------

The recommended way to install the library is by using GNU Autotools.

To install the library, just use the standard procedure:

./configure --prefix=PATH/TO/LIBXC
make
make check
make install

If you're not using a stable release tarball, you'll first need to
generate configure with:
autoreconf -i

CMake
-----

Support for CMake has also been recently contributed by Lori Burns.

The CMake file has the following caveats

* tested on Linux and Mac, static and shared lib, namespaced and
  non-namespaced headers, but really only to the extent that it works
  for Psi4
* all the fancy libtool options and Fortran interface _not_ tested
* test suite executed after build via `ctest`. But it has always
  totally passed or totally failed, which doesn't inspire confidence
* The generated `libxc_docs.txt` is large, and the generation step
  sometimes balks on it, leading to `xc_funcs.h` not found
  errors. Just execute again.

1) Building with CMake

Use the following procedure:

cmake -H. -Bobjdir
cd objdir && make
make test
make install

The build is also responsive to

  * static/shared toggle `BUILD_SHARED_LIBS`
  * install location `CMAKE_INSTALL_PREFIX`
  * namespacing of headers `NAMESPACE_INSTALL_INCLUDEDIR`
  * of course, `CMAKE_C_COMPILER`, `BUILD_TESTING`, and `CMAKE_C_FLAGS`

See CMakeLists.txt for options details. All these build options should
be passed as `cmake -DOPTION`.

2) Detecting with CMake

CMake builds install with `LibxcConfig.cmake`, `LibxcConfigVersion.cmake`, and
`LibxcTargets.cmake` files suitable for use with CMake
[`find_package()`](https://cmake.org/cmake/help/v3.2/command/find_package.html)
in `CONFIG` mode.

* `find_package(Libxc)` - find any xc libraries and headers

* `find_package(Libxc 3.0.0 EXACT CONFIG REQUIRED COMPONENTS static)`
  - find Libxc exactly version 3.0.0 built with static libraries or
  die trying

See cmake/LibxcConfig.cmake.in for details of how to detect the Config
file and what CMake variables and targets are exported to your
project.

3) Use with CMake

After `find_package(Libxc ...)`,

* test if package found with `if(${Libxc_FOUND})` or `if(TARGET
  Libxc::xc)`
* link to library (establishes dependency), including header and
  definitions configuration with `target_link_libraries(mytarget
  Libxc::xc)`
* include header files using `target_include_directories(mytarget
  PRIVATE $<TARGET_PROPERTY:Libxc::xc,INTERFACE_INCLUDE_DIRECTORIES>)`
* compile target applying `-DUSING_Libxc` definition using
  `target_compile_definitions(mytarget PRIVATE
  $<TARGET_PROPERTY:Libxc::xc,INTERFACE_COMPILE_DEFINITIONS>)`

FILE ORGANIZATION
=================

The distribution is organized as follows

./cmake      CMake helper files
./build      pkgconfig and Fedora spec files
./m4         m4 scripts used by configure.ac, and
             libxc.m4 used by other projects linking to libxc
./maple      the Maple source code for the functionals
./scripts    various scripts for libxc development
./src        source files
./testsuite  regression tests

The most important contents of the src directory for users are

xc.h         main header file with all external definitions
xc_funcs.h   automatically generated file with the list of functionals

In addition, developers will be interested in the following

util.h                         header file with internal definitions
*.f90 *.F90 xc_f.c string_f.h  Fortran 90 interface
*.f03 *.F03                    Fortran 2003 interface
funcs_*.c                      automatically generated files with
                               the functional definitions
functionals.c                  generic interface to simplify access
                               to the different families
lda.c gga.c mgga.c             interface to the different families of
                               functionals
special_functions.c            implementation of a series of special functions
hyb_gga_*.c                    definition of the different hybrid
                               GGA functionals
hyb_mgga_*.c                   definition of the different hybrid
                               meta-GGA functionals
lda_*.c                        definition of the different LDA functionals
gga_*.c                        definition of the different GGA functionals
mgga_*.c                       definition of the different meta-GGA functionals
work_lda.c                     code that simplifies the implementation of LDAs
work_gga_x.c                   code that simplifies the implementation of
                               exchange GGAs
work_gga_c.c                   code that simplifies the implementation of
                               some correlation GGAs
work_mgga_x.c                  code that simplifies the implementation of
                               exchange meta-GGAs
work_mgga_c.c                  code that simplifies the implementation of
                               some correlation meta-GGAs

Notes:

* Most functionals use the framework contained in a work_*.c
  file. This simplifies tremendously the implementation of the
  different functionals. The work_*.c are #include'd in the functional
  implementations through a preprocessor directive.

* Some files contain more than one functional, as similar functionals
  are usually grouped together. Therefore, the best way to find where
  a functional is implemented is by looking at its keyword in
  xc_funcs.h and using grep to find the correct file.

* The files where the functionals are defined are named as
  family_type_name.c, where: family - functional family (lda, gga,
  hyb_gga, or mgga) type - type of functional (x, c, xc, or k) name -
  name of the functional or class of functionals