Library usage

The interface is exposed through the gfnff_interface Fortran module and the gfnff_interface_c.h C header (located in include/). Two steps are required: initialise the calculator (topology setup) and call the singlepoint routine. The initialisation is typically the more expensive step; once complete, singlepoint evaluations can be called repeatedly on the same calculator object.

All coordinates are in Bohr; the energy is in Hartree, the gradient in Eh/Bohr, and sigma (3×3) is the stress tensor in Hartree (zero for non-periodic systems).

Fortran

use iso_fortran_env, only: real64
use gfnff_interface

type(gfnff_data) :: calc
integer  :: nat, ichrg, io
integer,  allocatable :: at(:)
real(real64), allocatable :: xyz(:,:), gradient(:,:)
real(real64) :: energy, sigma(3,3)

! ... populate nat, at, xyz, ichrg ...

call calc%init(nat, at, xyz, ichrg=ichrg, iostat=io)

call calc%singlepoint(nat, at, xyz, energy, gradient, iostat=io, sigma=sigma)

call calc%deallocate()

Full working example: app/main.F90.

C and C++

The same header serves both languages.

#include "gfnff_interface_c.h"

double sigma[3][3];   /* stress tensor (Hartree); zero for non-PBC */

c_gfnff_calculator calc =
    c_gfnff_calculator_init(nat, at, xyz, ichrg, printlevel, solvent);

c_gfnff_calculator_singlepoint(&calc, nat, at, xyz, &energy, gradient,
                               sigma, &iostat);

c_gfnff_calculator_deallocate(&calc);

Full working examples: test/main.c and test/main.cpp.

Periodic boundary conditions

PBC support is available via c_gfnff_calculator_init_pbc on the C/C++ side and via an optional lattice argument to calc%init in Fortran. See the PBC sections in test/main.c and test/main.cpp for worked examples.

Integrating into another project

CMake. Add the repository as a subdirectory and link against the exported target:

add_subdirectory(gfnff)
target_link_libraries(my_target PRIVATE gfnff)

Meson. Place the repository under subprojects/gfnff/ and wrap it:

gfnff_dep = dependency('gfnff', fallback: ['gfnff', 'gfnff_dep'])