! initialize tree data structure
        size(:,:) = -1.
        bnd_box(:,:) = 0.
        lrefine(:) = -1
        nodetype(:) = -1
        refine(:) = .FALSE.
        derefine(:) = .FALSE.
        parent(:,:) = -1
        child(:,:,:) = -1
        coord(:) = -1.
        neigh(:,:,:) = -1
        empty(:) = 0

! initialize solution array
        unk(:,:,:,:,:) = 0.