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#+TITLE: Code examples
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#+SETUPFILE: ../tools/theme.setup
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#+INCLUDE: ../tools/lib.org
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In this section, we present examples of usage of QMCkl.
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For simplicity, we assume that the wave function parameters are stores
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in a [[https://github.com/TREX-CoE/trexio][TREXIO]] file.
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* Checking errors
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All QMCkl functions return an error code. A convenient way to handle
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errors is to write an error-checking function that displays the
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error in text format and exits the program.
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#+NAME: qmckl_check_error
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#+begin_src f90
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subroutine qmckl_check_error(rc, message)
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use qmckl
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implicit none
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integer(qmckl_exit_code), intent(in) :: rc
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character(len=*) , intent(in) :: message
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character(len=128) :: str_buffer
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if (rc /= QMCKL_SUCCESS) then
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print *, message
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call qmckl_string_of_error(rc, str_buffer)
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print *, str_buffer
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call exit(rc)
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end if
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end subroutine qmckl_check_error
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#+end_src
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* Computing an atomic orbital on a grid
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:PROPERTIES:
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:header-args: :tangle ao_grid.f90
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:END:
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The following program, in Fortran, computes the values of an atomic
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orbital on a regular 3-dimensional grid. The 100^3 grid points are
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automatically defined, such that the molecule fits in a box with 5
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atomic units in the borders.
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This program uses the ~qmckl_check_error~ function defined above.
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To use this program, run
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#+begin_src bash :tangle no
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$ ao_grid <trexio_file> <AO_id> <point_num>
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#+end_src
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#+begin_src f90 :noweb yes
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<<qmckl_check_error>>
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program ao_grid
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use qmckl
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implicit none
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integer(qmckl_context) :: qmckl_ctx ! QMCkl context
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integer(qmckl_exit_code) :: rc ! Exit code of QMCkl functions
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character(len=128) :: trexio_filename
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character(len=128) :: str_buffer
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integer :: ao_id
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integer :: point_num_x
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integer(c_int64_t) :: nucl_num
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double precision, allocatable :: nucl_coord(:,:)
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integer(c_int64_t) :: point_num
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integer(c_int64_t) :: ao_num
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integer(c_int64_t) :: ipoint, i, j, k
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double precision :: x, y, z, dr(3)
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double precision :: rmin(3), rmax(3)
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double precision, allocatable :: points(:,:)
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double precision, allocatable :: ao_vgl(:,:,:)
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#+end_src
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Start by fetching the command-line arguments:
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#+begin_src f90
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if (iargc() /= 3) then
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print *, 'Syntax: ao_grid <trexio_file> <AO_id> <point_num>'
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call exit(-1)
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end if
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call getarg(1, trexio_filename)
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call getarg(2, str_buffer)
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read(str_buffer, *) ao_id
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call getarg(3, str_buffer)
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read(str_buffer, *) point_num_x
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if (point_num_x < 0 .or. point_num_x > 300) then
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print *, 'Error: 0 < point_num < 300'
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call exit(-1)
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end if
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#+end_src
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Create the QMCkl context and initialize it with the wave function
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present in the TREXIO file:
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#+begin_src f90
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qmckl_ctx = qmckl_context_create()
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rc = qmckl_trexio_read(qmckl_ctx, trexio_filename, 1_8*len(trim(trexio_filename)))
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call qmckl_check_error(rc, 'Read TREXIO')
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#+end_src
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Now we will compute the limits of the box in which the molecule fits.
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For that, we first need to ask QMCkl the coordinates of nuclei.
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#+begin_src f90
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rc = qmckl_get_nucleus_num(qmckl_ctx, nucl_num)
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call qmckl_check_error(rc, 'Get nucleus num')
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allocate( nucl_coord(3, nucl_num) )
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rc = qmckl_get_nucleus_coord(qmckl_ctx, 'N', nucl_coord, 3_8*nucl_num)
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call qmckl_check_error(rc, 'Get nucleus coord')
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#+end_src
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We now compute the coordinates of opposite points of the box, and
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the distance between points along the 3 directions:
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#+begin_src f90
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rmin(1) = minval( nucl_coord(1,:) ) - 5.d0
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rmin(2) = minval( nucl_coord(2,:) ) - 5.d0
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rmin(3) = minval( nucl_coord(3,:) ) - 5.d0
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rmax(1) = maxval( nucl_coord(1,:) ) + 5.d0
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rmax(2) = maxval( nucl_coord(2,:) ) + 5.d0
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rmax(3) = maxval( nucl_coord(3,:) ) + 5.d0
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dr(1:3) = (rmax(1:3) - rmin(1:3)) / dble(point_num_x-1)
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#+end_src
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We now produce the list of point coordinates where the AO will be
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evaluated:
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#+begin_src f90
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point_num = point_num_x**3
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allocate( points(point_num, 3) )
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ipoint=0
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z = rmin(3)
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do k=1,point_num_x
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y = rmin(2)
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do j=1,point_num_x
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x = rmin(1)
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do i=1,point_num_x
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ipoint = ipoint+1
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points(ipoint,1) = x
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points(ipoint,2) = y
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points(ipoint,3) = z
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x = x + dr(1)
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end do
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y = y + dr(2)
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end do
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z = z + dr(3)
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end do
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#+end_src
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We give the points to QMCkl:
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#+begin_src f90
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rc = qmckl_set_point(qmckl_ctx, 'T', points, point_num)
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call qmckl_check_error(rc, 'Setting points')
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#+end_src
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We allocate the space required to retrieve the values, gradients and
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Laplacian of all AOs, and ask to retrieve the values of the
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AOs computed at the point positions. For that, we first need to know
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the number of AOs:
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#+begin_src f90
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rc = qmckl_get_ao_basis_ao_num(qmckl_ctx, ao_num)
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call qmckl_check_error(rc, 'Getting ao_num')
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allocate( ao_vgl(ao_num, 5, point_num) )
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rc = qmckl_get_ao_basis_ao_vgl(qmckl_ctx, ao_vgl, ao_num*5_8*point_num)
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call qmckl_check_error(rc, 'Setting points')
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#+end_src
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We finally print the value of the AO:
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#+begin_src f90
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do ipoint=1, point_num
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print '(3(F16.10,X),E20.10)', points(ipoint, 1:3), ao_vgl(ao_id,1,ipoint)
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end do
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#+end_src
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#+begin_src f90
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deallocate( nucl_coord, points, ao_vgl )
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end program ao_grid
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#+end_src
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