node_worker Module Subroutine

   module subroutine node_worker(resources, sys_geom, method_config, calc_type)
      !! Node worker for processing fragments assigned by node coordinator
      !! Bond connectivity is accessed via sys_geom%bonds
      use mqc_error, only: error_t
      type(resources_t), intent(in) :: resources
      type(system_geometry_t), intent(in), optional :: sys_geom
      type(method_config_t), intent(in) :: method_config  !! Method configuration
      integer(int32), intent(in) :: calc_type

      integer(int64) :: fragment_idx
      integer(int32) :: fragment_size, dummy_msg
      integer(int32) :: fragment_type  !! 0 = monomer (indices), 1 = intersection (atom list)
      integer(int32), allocatable :: fragment_indices(:)
      type(calculation_result_t) :: result
      type(MPI_Status) :: status
      type(physical_fragment_t) :: phys_frag
      type(error_t) :: error

      ! MPI request handles for non-blocking operations
      type(request_t) :: req

      dummy_msg = 0

      do
         call isend(resources%mpi_comms%node_comm, dummy_msg, 0, TAG_WORKER_REQUEST, req)
         call wait(req)
         call irecv(resources%mpi_comms%node_comm, fragment_idx, 0, MPI_ANY_TAG, req)
         call wait(req, status)

         select case (status%MPI_TAG)
         case (TAG_WORKER_FRAGMENT)
            ! Receive fragment type (0 = monomer indices, 1 = intersection atom list)
            call irecv(resources%mpi_comms%node_comm, fragment_type, 0, TAG_WORKER_FRAGMENT, req)
            call wait(req)
            call irecv(resources%mpi_comms%node_comm, fragment_size, 0, TAG_WORKER_FRAGMENT, req)
            call wait(req)
            ! Note: must use blocking recv for allocatable arrays since size is unknown
            allocate (fragment_indices(fragment_size))
            call recv(resources%mpi_comms%node_comm, fragment_indices, 0, TAG_WORKER_FRAGMENT, status)

            ! Build physical fragment based on type
            if (present(sys_geom)) then
               if (fragment_type == 0) then
                  ! Monomer: fragment_indices are monomer indices
                  call build_fragment_from_indices(sys_geom, fragment_indices, phys_frag, error, sys_geom%bonds)
               else
                  ! Intersection: fragment_indices are atom indices
           call build_fragment_from_atom_list(sys_geom, fragment_indices, fragment_size, phys_frag, error, sys_geom%bonds)
               end if

               if (error%has_error()) then
                  call logger%error(error%get_full_trace())
                  call abort_comm(resources%mpi_comms%world_comm, 1)
               end if

               ! Process the chemistry fragment with physical geometry
          call do_fragment_work(fragment_idx, result, method_config, phys_frag, calc_type, resources%mpi_comms%world_comm)

               call phys_frag%destroy()
            else
               ! Process without physical geometry (old behavior)
               call do_fragment_work(fragment_idx, result, method_config, &
                                     calc_type=calc_type, world_comm=resources%mpi_comms%world_comm)
            end if

            ! Send result back to coordinator
            call result_isend(result, resources%mpi_comms%node_comm, 0, TAG_WORKER_SCALAR_RESULT, req)
            call wait(req)

            ! Clean up result
            call result%destroy()
            deallocate (fragment_indices)
         case (TAG_WORKER_FINISH)
            exit
         case default
            ! Unexpected MPI tag - this should not happen in normal operation
            call logger%error("Worker received unexpected MPI tag: "//to_char(status%MPI_TAG))
            call logger%error("Expected TAG_WORKER_FRAGMENT or TAG_WORKER_FINISH")
            call abort_comm(resources%mpi_comms%world_comm, 1)
         end select
      end do
   end subroutine node_worker