Interfacing TCHInt

For using the library, a cmake config file tchint-config.cmake is created in the build directory. By using the cmake find_package command, the tchint library can be made visible to cmake as a target tchint with corresponding include paths and linkage, e.g. by including:

find_package(tchint)
target_link_library(<my_target> tchint)

in the CMakeLists.txt of a calling program, we can link the target <my_target> to the tchint library. Note that this requires having the tchint build directory visible to cmake, which can be achieved e.g. by setting the environment variable TCHINT_ROOT=<tchint_build_dir>.

Alternatively, one can also manually link to the libtchint.so library. This then requires including the module directory <tchint_build_dir>/include when compiling, as the module files are required at compile time.

Exported functions

The direct interface is given in the form of the tchint module in src/tchint.F90 (or src/tchint.h for the C interface; note this is put into src/include of the build directory), which contains the interface functions for external code to call in order to: * set up the integrals * receive the value of an integral * receive two- or three-body matrix elements * finalize the library and deallocate resources

The corresponding functions’ signatures are:

!> Return a matrix element of a given excitation, i.e. between
!! a determinant D and a determinant excit * D (defined as replacing
!! excit(1,:) in D with excit(2,:)
!! This follows the slater condon rules for three-body terms
!> @param[in] D determinant from which the electrons are excited
!> @param[in] ex (2,lvl)-matrix with the ex(1,:) containing the
!!               source and ex(2,:) the target of the excitation
!!               as spin orbitals
!> @param[in] ex_parity parity of the excitation, i.e. if an even or
!!                      odd number of electrons are exchanged when
!!                      bringing the new determinant into canonical
!!                      order
function tc_matel(D, excit) result(matel)
  integer, intent(in) :: D(:)
  integer, intent(in) :: excit(:,:)
  real(dp) :: matel
end function

!> Initialize the tchint module. This pre-calculates all required
!! integrals and can thus take a substantial amount of time.
!! The settings are read from the tchint.json file.
subroutine tchint_init()
end subroutine

!> Finalize the tchint module, deallocating all resources
subroutine tchint_finalize()
end subroutine tchint_finalize

These functions can be called once the tchint module has been loaded with:

use tchint

An example program using the library is also availabe in examples/using_tchint.f90, with the corresponding examples/CMakeLists.txt for building the application.

The standalone program runs tchint_init followed by tchint_finalize. This precalculates all required integrals and writes them to disk. See also Running TCHInt calculations for a description of the functionality.