How to compile and link Libint
==============================


1. Introduction
---------------

The HFX modules in cp2k interface the C++ library Libint in order to calculate
the four center electron repulsion integrals. I you want to use that part of
the code, you have to compile and link in these libraries (libint.a and 
libderiv.a). 

Interfacing to the Libint library requires some additional attention due to the
fact that Fortran code has to inter-operate with some more sophisticated  C/C++
function prototypes such as arrays of pointers to functions.

In principle, the ISO_C_BINDING module, as a Fortran-2003 extension to the 
95 standard, provides an elegant way of handling interoperabilty with C or 
C++ code.
However, not all compilers come with full ISO_C_BINDING support and therefore 
all calls from cp2k to the Libint-library have to be redirected through a
c++ wrapper.


2. ISO_C_BINDING prerequisites
------------------------------

In order to compile CP2K and link to the Libint library without the need of 
additional wrappers, a Fortran Compiler needs to support the following
intrinsic datatypes and procedures:

 - Interoperability types: C_INT, C_DOUBLE, C_PTR
 - Procedures: C_F_POINTER, C_LOC and C_F_PROCPOINTER

If all these features are present your compiler should be able to compile 
cp2k and link to Libint without any problems. (For a list of compilers
with full support see section 5). If one or more of the above features are
missing, you have to compile cp2k with the flag -D__HAS_NO_ISO_C_BINDING and 
additionally have to provide the wrapper.

Note: By default, cp2k assumes full ISO_C_BINDING capabilities for the 
      compilers.

Section 4 explains how to proceed in both cases.


3. How to compile Libint
------------------------

Compilation and installation of Libint is relatively straightforward. 
You can download the library from

http://www.files.chem.vt.edu/chem-dept/valeev/software/libint/download.html

(Note: Do not use Libint 1.1.3. All what follows has been tested for 
       version 1.1.4.)

Once you have unzipped the tarball you just execute the following commands

   ./configure --prefix='your_install_directory'
   make
   make install

In that case, the library is compiled with default settings for the angular
momentum (i.e. g-functions for energies, f-functions for derivatives).
If you want to use basis functions with higher angular momenta, you have to
provide this information to the configure script. For example:

   ./configure --prefix='your_install_directory' --with-libint-max-am=5 --with-libderiv-max-am1=4

for h- and g-functions respectively.

NOTE: If you do not use the default settings, you have to specify some 
      preprocessor flags in the cp2k arch file. (See section 4.3 for details.)


4. How to link to Libint
------------------------

If you want to link against the Libint library, you have to add -D__LIBINT
to the DFLAGS in the cp2k architecture file. If you try to run cp2k using
the Hartree-Fock module without specifying this flag, you will get a 
run-time error message and cp2k will abort.


4.1 Compilers that fully support the ISO_C_BINDING standard
-----------------------------------------------------------

In that case you only have to link in the Libint libraries in the correct 
order via the arch file:

LIBS        = [...]\
              'path_to_libint_libs'/lib/libderiv.a\
              'path_to_libint_libs'/lib/libint.a\
              -lstdc++

lstdc++ is needed if you use the GNU C++ compiler.


4.2 Compilers that do not fully support the ISO_C_BINDING standard
------------------------------------------------------------------

In that case, you have to compile the C++ wrapper:

	1. Move into cp2k/tools/hfx_tools/libint_tools
        2. The source file is called libint_cpp_wrapper.cpp
        3. Compile this file with your favourite C++ compiler (here gcc)

             g++ -O3 -c libint_cpp_wrapper.cpp -I/'path_to_libint_include'

           in order to obtain libint_cpp_wrapper.o. (Note: Do not forget the
           optimization flag!)
           

For the compilation of cp2k you have to add the following DFLAG to your 
arch file:
        
        -D__HAS_NO_ISO_C_BINDING

And in the LIB section, you add
       
LIBS        = [...]\
              /cp2k/tools/hfx_tools/libint_tools/libint_cpp_wrapper.o\
              'path_to_libint_libs'/lib/libderiv.a\
              'path_to_libint_libs'/lib/libint.a\
               -lstdc++

Note: Take care of the correct order!


4.3 Libint with non-default angular momentum settings
-----------------------------------------------------

In that case you have to specify two additional DFLAGS that tell cp2k 
what is the maximum angular momentum that the libint libraries can deal
with. For h-functions energies and g-functions derivatives for example,
that is

-  -D__LIBINT_MAX_AM=6
_  -D__LIBDERIV_MAX_AM1=5
           
Note: This is max_am + 1 as specified in the configure script. These values can
      also be found in libint.h and lideriv.h respectively.


5. Appendix
-----------

5.1 Compilers that support full ISO_C_BINDING
---------------------------------------------

- gfortran 4.4.1
- intel 11.1.056
- crayftn 7.1.2
- xlf 11.1 (compiles, but bug in execution) 


5.2 Known issues
----------------

5.2.1 Underscore problem
------------------------

- xlf compilers require special care for the linking of the wrapper. Add

  FCFLAGS = [...]\
            -qextname=wrapper_build_deriv1_eri:wrapper_build_eri:wrapper_free_libderiv:wrapper_free_libint:wrapper_init_deriv:wrapper_init_lib

- other compilers might need -fno-second-underscore or similar flags in order
  to avoid problems

- g95 needs two trailing underscores instead of one as provided in the file

5.2.2 Cross compilation
-----------------------

Most HPC facilities offer different architectures for login and compute nodes.
Take care to compile libint and the wrapper with the correct C/C++ compiler.
For PGI on a CRAY-XT3 for example 

   ./configure --prefix='your_install_directory' --with-cxx=CC --target=cray --host=x86_64-unknown-linux-gnu

and 

   CC -c -target=catamount libint_cpp_wrapper.cpp -I/'path_to_libint.h_and_libderiv.h'

5.3 Upgrading from a previous cp2k version
------------------------------------------

If you already have and older cp2k arch file that worked with your 
compiler/hardware, the only thing you have to do is either to add the
-D__HAS_NO_ISO_C_BINDING flag or not.
