Improved the accuracy of delta sum close to the edge of the Fermi window by including energy eigenvalues on-the-fly of the tetrahedral vertices with energies lying outside the chosen Fermi-window and added a test for it

[prikarsartam]

Following Issue #99 this commit makes the following improvements to resolve it across the computation of electronic spectrum using elphbolt. In summary:

• Added a way to keep track of the exceptional vertices during analytical tetrahedron method.
• Added a new subroutine to compute the eigenvalues of the tetrahedral vertices on-the-fly.
• Updated Fermi-window adjustment across the computation of electronic spectral properties.
• Added a test to compute the electronic density of states with Fermi-window adjusted analytical tetrahedron method.
  (fpm test fermi_window_adjustment --runner="sh test/3C-SiC/fpm_run_fermi_window_adj_caf.sh")

---

1. Update in form_tetrahedra_3d in src/delta.f90 : to keep track of the vertices whose eigenvalues lie outside the chosen Fermi-window by storing their indices as negative.

Previously:

if(blocks) then
                !Which point in indexlist does aux correspond to?
                call binsearch(indexlist, aux, tmp) !tmp < 0 if search fails.
             end if
             tetra(count, tl) = tmp 

is updated to:

! If energy-restricted blocks are being used, find the index in indexlist
	    ! The updated snippet keeps track of vertices whose eigenvalues lie outside the chosen window around fermi energy
	    
	    if(blocks) then
		call binsearch(indexlist, aux, tmp)   ! Binary search in indexlist

		
		if (tmp < 0) then
		    tetra(count, tl) = -aux            ! If vertex is outside Fermi window, save negative index (for the exceptional vertices)
		else
		    tetra(count, tl) = tmp             ! Save the index from indexlist
		end if
		
	    else
	    tetra(count, tl) = aux                ! Save the multiplexed index
	    end if

---

2. Added a new subroutine fermi_window_adjusted_fill_tetrahedra_3d in src/delta.f90 to compute the eigenvalues of the vertices on-the-fly.

subroutine fermi_window_adjusted_fill_tetrahedra_3d(wann, crys, wave_vector_mesh, tetra, evals, tetra_evals)
    type(wannier), intent(in) :: wann
    type(crystal), intent(in) :: crys
    integer(i64), intent(in) :: tetra(:,:)
    real(r64), intent(in) :: evals(:,:)
    integer(i64), intent(in) :: wave_vector_mesh(1, 3)
    
    real(r64), allocatable, intent(out) :: tetra_evals(:,:,:)

    integer(i64) :: iv, it, ib, numbands, aux, numtetra
    integer(i64) :: kpoint(3)
    
    real(r64), allocatable :: energies(:,:)
    real(r64) :: kvecs(1,3)

    ! Calculate dimensions
    numtetra = size(tetra, 1)
    numbands = size(evals, 2)

    print *, "Number of tetrahedra (numtetra) =", numtetra
    print *, "Number of bands (numbands) =", numbands

	  print *, "tetra shape:", shape(tetra)
	  print *, "evals shape:", shape(evals)
	  print *, "tetra_evals shape:", shape(tetra_evals)

    ! Allocate arrays with correct dimensions
   allocate(tetra_evals(numtetra, numbands, 4))
   allocate(energies(1, wann%numwannbands))

    ! Initialize tetra_evals and energies
    tetra_evals(:,:,:) = 0.0_r64
    energies(:, :) = 0.0_r64

    ! Loop over all tetrahedra and vertices
    do it = 1, numtetra
       do iv = 1, 4
          aux = tetra(it, iv)
          
          !print *, "Accessing tetra array with index it=", it, ", iv=", iv

	  !print *, "aux = ", aux

          if (aux < 0) then
             ! Vertex outside Fermi window, calculate on-the-fly
             call demux_vector(-aux, kpoint, wave_vector_mesh, 1_i64)
             kvecs(1, :) = real(kpoint) / real(wave_vector_mesh(1, :))

             call wann%el_wann(crys, 1_i64, kvecs, energies)
             tetra_evals(it, :, iv) = energies(1, :)
          else
             ! Use pre-calculated eigenvalue
             tetra_evals(it, :, iv) = evals(aux, :)
          end if
       end do
    end do

    ! Sort eigenvalues for each tetrahedron vertex
    do it = 1, numtetra
       do ib = 1, numbands
          call sort(tetra_evals(it, ib, :))
       end do
    end do
    deallocate(energies)
end subroutine fermi_window_adjusted_fill_tetrahedra_3d

And made it public.

---

3. Implemented the fermi-window adjustment in src/electron.f90 in computation of electronic spectral properties, by calling fermi_window_adjusted_fill_tetrahedra_3d instead of fill_tetrahedra_3d; which remains as it is for computing phononic spectral properties.

NOTE: that these changes ended up compiling and installing fine, but caused the electron DOS test to fail. Then I found out that it compares two computed and reference sets of values within a tolerance of $$10^{-11}$$, so I had printed out the numbers and found them different. So for the new test of this improvement, I took the newly generated data from the el.dos file and put them in the same syntax as before inside test_array(itest)%assert in my test file test/check_fermi_window_adjustment.f90. That way, as expected, it passes.

---

4. Finally added the test.

• Added test/check_fermi_window_adjustment.f90 similar to bte_regression.f90, except with minimum dependencies to test the computation of electronic density of states.
• Added test/3C-SiC/fpm_run_fermi_window_adj_caf.sh similar to the runner of bte_regression test which executes cafrun -n 2 ../build/caf_*/test/fermi_window_adjustment | tee 3C_SiC_test.output.
• Updated fpm.toml with an addition of

[[test]]
name = "fermi_window_adjustment"
source-dir="test"
main = "check_fermi_window_adjustment.f90"

---

With these, the fermi-window adjustment can be easily tested by running

fpm test fermi_window_adjustment --runner="sh test/3C-SiC/fpm_run_fermi_window_adj_caf.sh".

Which renders the final output that ends with the following:

___________________________________________________________________________
___________________________________________Calculating density of states...
Calculating electron density of states...
 electron DOS =>  PASSED! :)
 +--------------------------------------------------+
  Tests carried out: [number of symmetries; symmetry group; electron energies; electron DOS]
  Total number of tests:            4
  Number of tests passed:            4
 +--------------------------------------------------+
..............
| Timing info: elphbolt: Fermi-window adjustment  0.12848908E-03 hr
..............

---

Therefore it Fixes #99

[nakib]

Dear Pritam,

Thanks a lot for your contribution. Really appreciate it!

I left my review below. Please take a look when you have time.

[prikarsartam]

Dear Nakib,

Thank you for all the valuable comments. I have addressed all the suggestions, made the required changes and removed the new test file I added.

Kindly review the latest implementations when you are available.

Thank you.

[nakib]

Dear Pritam,

Thanks again for your contribution. Really appreciate it.

Best,
Nakib

[prikarsartam]

Dear Nakib,

I am very happy to see my little contribution merged in elphbolt. It will be my pleasure to work on it in the future.

Thank you,
Pritam.