mqc_method_dft.f90 Source File

!! Density Functional Theory (DFT) method implementation for metalquicha
module mqc_method_dft
   !! Implements Kohn-Sham DFT quantum chemistry method
   !! Provides energy and gradient calculations using self-consistent field
   !! with exchange-correlation functionals.
   use pic_types, only: dp
   use mqc_method_base, only: qc_method_t
   use mqc_result_types, only: calculation_result_t
   use mqc_physical_fragment, only: physical_fragment_t
   implicit none
   private

   public :: dft_method_t, dft_options_t

   type :: dft_options_t
      !! DFT calculation options
      character(len=32) :: basis_set = 'sto-3g'
         !! Basis set name
      character(len=32) :: functional = 'b3lyp'
         !! Exchange-correlation functional
      integer :: max_iter = 100
         !! Maximum SCF iterations
      real(dp) :: energy_tol = 1.0e-8_dp
         !! Energy convergence threshold
      real(dp) :: density_tol = 1.0e-6_dp
         !! Density matrix convergence threshold
      logical :: spherical = .true.
         !! Use spherical (true) or Cartesian (false) basis
      logical :: verbose = .false.
         !! Print SCF iterations

      ! Grid settings
      character(len=16) :: grid_type = 'medium'
         !! Integration grid quality
      integer :: radial_points = 75
         !! Number of radial grid points per atom
      integer :: angular_points = 302
         !! Number of angular grid points (Lebedev)

      ! Density fitting
      logical :: use_density_fitting = .false.
         !! Use RI-J approximation
      character(len=32) :: aux_basis_set = ''
         !! Auxiliary basis set for density fitting

      ! Dispersion correction
      logical :: use_dispersion = .false.
         !! Add empirical dispersion correction
      character(len=8) :: dispersion_type = 'd3bj'
         !! Dispersion type: "d3", "d3bj", "d4"

      ! DIIS acceleration
      logical :: use_diis = .true.
         !! Use DIIS for SCF convergence
      integer :: diis_size = 8
         !! Number of Fock matrices in DIIS
   end type dft_options_t

   type, extends(qc_method_t) :: dft_method_t
      !! DFT method implementation
      !!
      !! Kohn-Sham DFT with configurable exchange-correlation functional,
      !! integration grid, and optional density fitting.
      type(dft_options_t) :: options
   contains
      procedure :: calc_energy => dft_calc_energy
      procedure :: calc_gradient => dft_calc_gradient
      procedure :: calc_hessian => dft_calc_hessian
   end type dft_method_t

contains

   subroutine dft_calc_energy(this, fragment, result)
      !! Calculate electronic energy using Kohn-Sham DFT
      !!
      !! TODO: Implementation requires:
      !! 1. Build basis set from fragment geometry
      !! 2. Compute one-electron integrals (S, T, V)
      !! 3. Set up integration grid for XC
      !! 4. Initial density guess (SAD or core Hamiltonian)
      !! 5. SCF iterations:
      !!    a. Build Coulomb matrix J
      !!    b. Evaluate XC energy and potential on grid
      !!    c. Build Fock matrix F = H + J + Vxc
      !!    d. Apply DIIS if enabled
      !!    e. Diagonalize F -> new density
      !!    f. Check convergence
      !! 6. Compute final energy
      class(dft_method_t), intent(in) :: this
      type(physical_fragment_t), intent(in) :: fragment
      type(calculation_result_t), intent(out) :: result

      if (this%options%verbose) then
         print *, "DFT: Calculating energy using ", trim(this%options%functional)
         print *, "DFT: Basis set: ", trim(this%options%basis_set)
         print *, "DFT: Fragment has", fragment%n_atoms, "atoms"
         print *, "DFT: nelec =", fragment%nelec
         print *, "DFT: charge =", fragment%charge
         print *, "DFT: Grid type: ", trim(this%options%grid_type)
         if (this%options%use_density_fitting) then
            print *, "DFT: Using density fitting (RI-J)"
         end if
         if (this%options%use_dispersion) then
            print *, "DFT: Dispersion correction: ", trim(this%options%dispersion_type)
         end if
      end if

      ! Placeholder: Return dummy energy
      ! TODO: Implement actual DFT calculation
      result%energy%scf = -1.0_dp*fragment%n_atoms  ! Placeholder
      result%has_energy = .true.

      if (this%options%verbose) then
         print *, "DFT: [STUB] Energy =", result%energy%total()
      end if

   end subroutine dft_calc_energy

   subroutine dft_calc_gradient(this, fragment, result)
      !! Calculate energy gradient using Kohn-Sham DFT
      !!
      !! TODO: Implementation requires:
      !! 1. Converged SCF (call calc_energy to get density)
      !! 2. Compute gradient contributions:
      !!    a. One-electron integral derivatives
      !!    b. Two-electron integral derivatives (or RI-J)
      !!    c. XC potential derivatives (grid-based)
      !!    d. Overlap derivative (Pulay force)
      !! 3. Optional: dispersion gradient
      class(dft_method_t), intent(in) :: this
      type(physical_fragment_t), intent(in) :: fragment
      type(calculation_result_t), intent(out) :: result

      if (this%options%verbose) then
         print *, "DFT: Calculating gradient using ", trim(this%options%functional)
      end if

      ! First get energy (and converged density)
      call this%calc_energy(fragment, result)

      ! Allocate and fill dummy gradient
      allocate (result%gradient(3, fragment%n_atoms))
      result%gradient = 0.0_dp  ! Placeholder
      result%has_gradient = .true.

      if (this%options%verbose) then
         print *, "DFT: [STUB] Gradient computed"
      end if

   end subroutine dft_calc_gradient

   subroutine dft_calc_hessian(this, fragment, result)
      !! Calculate energy Hessian using Kohn-Sham DFT
      !!
      !! TODO: Analytical Hessian requires:
      !! 1. Converged SCF and gradient
      !! 2. Coupled-perturbed Kohn-Sham (CPKS) equations
      !! 3. Second derivatives of integrals
      !! 4. XC kernel contributions
      !! Alternative: Use finite difference of gradients (via driver)
      class(dft_method_t), intent(in) :: this
      type(physical_fragment_t), intent(in) :: fragment
      type(calculation_result_t), intent(out) :: result

      if (this%options%verbose) then
         print *, "DFT: Analytical Hessian not yet implemented"
         print *, "DFT: Use finite difference of gradients instead"
      end if

      ! For now, just compute energy
      call this%calc_energy(fragment, result)
      result%has_hessian = .false.

   end subroutine dft_calc_hessian

end module mqc_method_dft