open Util
open Lacaml.D
open Bigarray

let cutoff = Constants.cutoff 
let cutoff2 = cutoff *. cutoff

exception NullQuartet
exception Found

let at_least_one_valid arr =
  try
    Array.iter (fun x -> if (abs_float x > cutoff) then raise Found) arr ; false
  with Found -> true


(** Horizontal and Vertical Recurrence Relations (HVRR) *)
let hvrr_two_e_vector (angMom_a, angMom_b, angMom_c, angMom_d)
    (totAngMom_a, totAngMom_b, totAngMom_c, totAngMom_d)
    (maxm, zero_m_array)
    (expo_b, expo_d)
    (expo_inv_p, expo_inv_q)
    (center_ab, center_cd, center_pq)
    map_1d map_2d np nq
  =

  let totAngMom_a = Angular_momentum.to_int totAngMom_a
  and totAngMom_b = Angular_momentum.to_int totAngMom_b
  and totAngMom_c = Angular_momentum.to_int totAngMom_c
  and totAngMom_d = Angular_momentum.to_int totAngMom_d
  in

  (** Vertical recurrence relations *)
  let rec vrr0_v m angMom_a = function
    | 0 -> Some zero_m_array.(m)
    | totAngMom_a ->
      let key = Zkey.of_int_tuple (Zkey.Three angMom_a)
      in

      try Zmap.find map_1d.(m) key with
      | Not_found -> 
        let result = 
          let am, amm, amxyz, xyz  =
            match angMom_a with
            | (x,0,0) -> (x-1,0,0),(x-2,0,0), x-1, 0
            | (x,y,0) -> (x,y-1,0),(x,y-2,0), y-1, 1
            | (x,y,z) -> (x,y,z-1),(x,y,z-2), z-1, 2
          in
          if amxyz < 0 then 
            None
          else
            begin
              let cab = Coordinate.coord center_ab xyz in
              let v1_top, p1_top = 
                if abs_float cab < cutoff then
                  None,
                  vrr0_v (m+1) am (totAngMom_a-1)
                else
                  vrr0_v m am (totAngMom_a-1), 
                  vrr0_v (m+1) am (totAngMom_a-1)
              in
              let v1_top2, p1_top2 = 
                if amxyz < 1 then (None,None) else
                  vrr0_v m amm (totAngMom_a-2), 
                  vrr0_v (m+1) amm (totAngMom_a-2) 
              in

              let result = Array.make_matrix np nq 0. in
              if amxyz < 1 then
                begin
                  let p0 = 
                    match p1_top with
                    | Some p1_top -> p1_top
                    | _ -> assert false
                  in
                  begin
                    match v1_top with
                    | None -> ()
                    | Some v0 ->
                      for l=0 to np-1 do
                        let f0 = 
                          -. expo_b.(l) *. expo_inv_p.(l) *. cab
                        in
                        for k=0 to nq-1 do
                          result.(l).(k) <- v0.(l).(k) *. f0
                        done
                      done
                  end;
                  for l=0 to np-1 do
                    for k=0 to nq-1 do
                      result.(l).(k) <- result.(l).(k)
                        +. expo_inv_p.(l) *. center_pq.(xyz).(l).(k) *. p0.(l).(k)
                    done
                  done
                end
              else
                begin
                  let p0 = 
                    match p1_top with
                    | Some p1_top -> p1_top
                    | _ -> assert false
                  in
                  begin
                    match v1_top with
                    | None -> ()
                    | Some v0 ->
                      for l=0 to np-1 do
                        let f0 = -. expo_b.(l) *. expo_inv_p.(l) *. cab in
                        for k=0 to nq-1 do
                          result.(l).(k) <- v0.(l).(k) *. f0
                        done
                      done
                  end;
                  let v1 =
                    match v1_top2 with
                    | Some v1_top2 -> v1_top2
                    | None -> assert false
                  in
                  let v2 = 
                    match p1_top2 with
                    | Some p1_top2 -> p1_top2
                    | None -> assert false
                  in
                  for l=0 to np-1 do
                    let f = (float_of_int amxyz) *. expo_inv_p.(l) *. 0.5 in
                    for k=0 to nq-1
                    do
                      result.(l).(k) <- result.(l).(k) +.
                        expo_inv_p.(l) *. center_pq.(xyz).(l).(k) *. p0.(l).(k) +.
                        f *. (v1.(l).(k) +. v2.(l).(k) *. expo_inv_p.(l)) 
                    done
                done
              end;
              Some result 
            end
          in
          Zmap.add map_1d.(m) key result;
          result

  and vrr_v m angMom_a angMom_c totAngMom_a totAngMom_c =

    match (totAngMom_a, totAngMom_c) with
    | (i,0) -> vrr0_v m angMom_a totAngMom_a 
    | (_,_) ->

      let key =  Zkey.of_int_tuple (Zkey.Six (angMom_a, angMom_c))
      in

      try Zmap.find map_2d.(m) key with
      | Not_found -> 
        let result = 
        begin
            let am, cm, cmm, axyz, cxyz, xyz =
              let (aax, aay, aaz) = angMom_a 
              and (acx, acy, acz) = angMom_c in
              if (acz > 0) then
                (aax, aay, aaz-1),
                (acx, acy, acz-1),
                (acx, acy, acz-2),
                aaz, acz, 2
              else if (acy > 0) then
                (aax, aay-1,aaz),
                (acx, acy-1,acz),
                (acx, acy-2,acz),
                aay,acy, 1
              else
                (aax-1,aay,aaz),
                (acx-1,acy,acz),
                (acx-2,acy,acz),
                aax,acx, 0
            in
            (*
            let result = Array.make_matrix np nq 0. in
            *)
            let do_compute = ref false in
            let v1 =
              let f = -. (Coordinate.coord center_cd xyz) in

              let f1 =
                Array.init nq (fun k ->
                  let x = expo_d.(k) *. expo_inv_q.(k) *. f in
                  if ( (not !do_compute) && (abs_float x > cutoff) ) then
                    do_compute := true;
                  x)
              in
              if (!do_compute) then
                match vrr_v m angMom_a cm totAngMom_a (totAngMom_c-1) with
                | None -> None
                | Some v1 -> 
                    begin
                      let result = Array.make_matrix np nq 0. in
                      for l=0 to np-1 do
                        for k=0 to nq-1 do
                          result.(l).(k) <- v1.(l).(k) *. f1.(k)
                        done
                      done;
                      Some result
                    end
              else None
            in

            let v2 =
              let f2 =
                Array.init np (fun l ->
                  Array.init nq (fun k ->
                    let x = expo_inv_q.(k) *. center_pq.(xyz).(l).(k) in
                    if (!do_compute) then x
                    else (if abs_float x > cutoff then do_compute := true ; x)
                ) )
              in
              if (!do_compute) then
                match vrr_v (m+1) angMom_a cm totAngMom_a (totAngMom_c-1) with
                | None -> None
                | Some v2 -> 
                    begin
                      for l=0 to np-1 do
                        for k=0 to nq-1 do
                          f2.(l).(k) <- -. v2.(l).(k) *. f2.(l).(k)
                        done
                      done;
                      Some f2
                    end
              else
                None
            in

            let p1 = 
              match v1, v2 with
              | None, None -> None
              | None, Some v2 -> Some v2
              | Some v1, None -> Some v1
              | Some v1, Some v2 ->
                  begin
                    for l=0 to np-1 do
                      for k=0 to nq-1 do
                        v2.(l).(k) <- v2.(l).(k) +. v1.(l).(k)
                      done
                    done;
                    Some v2
                  end
            in

            let p2 = 
              if cxyz < 2 then p1 else
                let fcm = (float_of_int (cxyz-1)) *. 0.5 in
                let f1 =
                  Array.init nq (fun k ->
                    let x = fcm *. expo_inv_q.(k) in
                    if (!do_compute) then x
                    else (if abs_float x > cutoff then do_compute := true ; x)
                  )
                in
                let v1 = 
                  if (!do_compute) then
                    match vrr_v m angMom_a cmm totAngMom_a (totAngMom_c-2) with
                    | None -> None
                    | Some v1 -> 
                        begin
                          let result = Array.make_matrix np nq 0. in
                          for l=0 to np-1 do
                            for k=0 to nq-1 do
                              result.(l).(k) <- v1.(l).(k) *. f1.(k)
                            done;
                          done;
                          Some result
                        end
                  else None
                in

                let v3 = 
                  let f2 = 
                    Array.init nq (fun k -> 
                      let x = expo_inv_q.(k) *. f1.(k) in
                      if (!do_compute) then x
                      else (if abs_float x > cutoff then do_compute := true ; x)
                    )
                  in
                  if (!do_compute) then
                    match vrr_v (m+1) angMom_a cmm totAngMom_a (totAngMom_c-2) with
                    | None -> None
                    | Some v3 -> 
                        begin
                          let result = Array.make_matrix np nq 0. in
                          for l=0 to np-1 do
                            for k=0 to nq-1 do
                              result.(l).(k) <- v3.(l).(k) *. f2.(k)
                            done
                          done;
                          Some result
                        end
                  else None
                in
                match p1, v1, v3 with
                | None, None, None    -> None
                | Some p1, None, None -> Some p1
                | None, Some v1, None -> Some v1
                | None, None, Some v3 -> Some v3
                | Some p1, Some v1, Some v3 ->
                    begin
                      for l=0 to np-1 do
                        for k=0 to nq-1 do
                          v3.(l).(k) <- p1.(l).(k) +. v1.(l).(k) +. v3.(l).(k)
                        done
                      done;
                      Some v3
                    end
                | Some p1, Some v1, None ->
                    begin
                      for l=0 to np-1 do
                        for k=0 to nq-1 do
                          p1.(l).(k) <- v1.(l).(k) +. p1.(l).(k)
                        done
                      done;
                      Some p1
                    end
                | Some p1, None, Some v3 ->
                    begin
                      for l=0 to np-1 do
                        for k=0 to nq-1 do
                          p1.(l).(k) <- p1.(l).(k) +. v3.(l).(k)
                        done
                      done;
                      Some p1
                    end
                | None   , Some v1, Some v3 ->
                    begin
                      for l=0 to np-1 do
                        for k=0 to nq-1 do
                          v3.(l).(k) <- v1.(l).(k) +. v3.(l).(k)
                        done
                      done;
                      Some v3
                    end
            in
            if (axyz < 1) || (cxyz < 1) then p2 else
              let v = 
                vrr_v (m+1) am cm (totAngMom_a-1) (totAngMom_c-1) 
              in
              match (p2, v) with
              | None, None   -> None
              | Some p2, None -> Some p2
              | _, Some v ->
                  begin
                    let p2 =
                      match p2 with
                      | None -> Array.make_matrix np nq 0.
                      | Some p2 -> p2
                    in
                    for l=0 to np-1 do
                      let fa = (float_of_int axyz) *. expo_inv_p.(l) *. 0.5 in
                      for k=0 to nq-1 do
                        p2.(l).(k) <- p2.(l).(k) -. fa *. expo_inv_q.(k) *. v.(l).(k)
                      done
                    done;
                    Some p2
                  end
          end
        in Zmap.add map_2d.(m) key result;
        result




  (** Horizontal recurrence relations *)
  and hrr0_v angMom_a angMom_b angMom_c 
      totAngMom_a totAngMom_b totAngMom_c =

    match totAngMom_b with
    | 0 ->
      begin
        match (totAngMom_a, totAngMom_c) with
        | (0,0) -> Array.fold_left (fun accu c ->
              accu +. Array.fold_left (+.) 0. c) 0. zero_m_array.(0)
        | (_,_) ->
          begin
            match vrr_v 0 angMom_a angMom_c totAngMom_a totAngMom_c with
            | Some matrix -> Array.fold_left (fun accu c -> accu +. Array.fold_left (+.) 0. c) 0. matrix
            | None -> 0.
          end
      end
    | 1 ->
      let (aax, aay, aaz) = angMom_a in
      let ap, xyz =
        match angMom_b with
        | (_,_,1) -> (aax,aay,aaz+1), 2
        | (_,1,_) -> (aax,aay+1,aaz), 1
        | (_,_,_) -> (aax+1,aay,aaz), 0
      in
      let f = Coordinate.coord center_ab xyz in
      let v1 = 
        match vrr_v 0 ap angMom_c (totAngMom_a+1) totAngMom_c with
        | Some matrix -> Array.fold_left (fun accu c -> accu +. Array.fold_left (+.) 0. c) 0. matrix
        | None -> 0.
      in
      if (abs_float f < cutoff) then v1 else
        let v2 =
        match vrr_v 0 angMom_a angMom_c totAngMom_a totAngMom_c with 
        | Some matrix -> Array.fold_left (fun accu c -> accu +. Array.fold_left (+.) 0. c) 0. matrix
        | None -> 0.
        in
        v1 +. v2 *. f
    | _ ->
      let (aax, aay, aaz) = angMom_a
      and (abx, aby, abz) = angMom_b in
      let bxyz, xyz =
        match angMom_b with
        | (0,0,_) -> abz, 2
        | (0,_,_) -> aby, 1
        | _       -> abx, 0
      in
      if (bxyz < 1) then 0. else
        let ap, bm =
          match xyz with
          | 0 -> (aax+1,aay,aaz),(abx-1,aby,abz)
          | 1 -> (aax,aay+1,aaz),(abx,aby-1,abz)
          | _ -> (aax,aay,aaz+1),(abx,aby,abz-1)
        in

        let h1 = 
          hrr0_v ap bm angMom_c (totAngMom_a+1) (totAngMom_b-1) totAngMom_c 
        in
        let f = (Coordinate.coord center_ab xyz) in
        if (abs_float f < cutoff) then h1 else
          let h2 = 
            hrr0_v angMom_a bm angMom_c totAngMom_a (totAngMom_b-1) totAngMom_c 
          in
          h1 +. h2 *. f

  and hrr_v angMom_a angMom_b angMom_c angMom_d
      totAngMom_a totAngMom_b totAngMom_c totAngMom_d =

    match (totAngMom_b, totAngMom_d) with
    | (_,0) -> if (totAngMom_b = 0) then
        begin
          match vrr_v 0 angMom_a angMom_c totAngMom_a totAngMom_c with
          | Some matrix -> Array.fold_left (fun accu c -> accu +. Array.fold_left (+.) 0. c) 0. matrix
          | None -> 0.
        end
      else
        hrr0_v angMom_a angMom_b angMom_c totAngMom_a totAngMom_b totAngMom_c
    | (_,_) ->
      let (acx, acy, acz) = angMom_c
      and (adx, ady, adz) = angMom_d in
      let cp, dm, xyz =
        match angMom_d with
        | (_,0,0) -> (acx+1, acy, acz), (adx-1, ady, adz), 0
        | (_,_,0) -> (acx, acy+1, acz), (adx, ady-1, adz), 1
        | _       -> (acx, acy, acz+1), (adx, ady, adz-1), 2
      in
      let h1 = 
        hrr_v angMom_a angMom_b cp dm totAngMom_a totAngMom_b (totAngMom_c+1) (totAngMom_d-1)
      in
      let f = (Coordinate.coord center_cd xyz) in
      if (abs_float f < cutoff) then
        h1
      else
        let h2 =
          hrr_v angMom_a angMom_b angMom_c dm totAngMom_a totAngMom_b totAngMom_c (totAngMom_d-1)
        in h1 +. f *. h2
  in
  hrr_v 
      (angMom_a.(0),angMom_a.(1),angMom_a.(2))
      (angMom_b.(0),angMom_b.(1),angMom_b.(2))
      (angMom_c.(0),angMom_c.(1),angMom_c.(2))
      (angMom_d.(0),angMom_d.(1),angMom_d.(2))
      totAngMom_a totAngMom_b totAngMom_c totAngMom_d 







let contracted_class_shell_pairs ~zero_m ?schwartz_p ?schwartz_q shell_p shell_q : float Zmap.t =

  let shell_a = shell_p.ContractedShellPair.shell_a
  and shell_b = shell_p.ContractedShellPair.shell_b
  and shell_c = shell_q.ContractedShellPair.shell_a
  and shell_d = shell_q.ContractedShellPair.shell_b
  and sp = shell_p.ContractedShellPair.shell_pairs
  and sq = shell_q.ContractedShellPair.shell_pairs
  in
  let maxm = 
    shell_p.ContractedShellPair.totAngMomInt +
    shell_q.ContractedShellPair.totAngMomInt 
  in

  (* Pre-computation of integral class indices *)
  let class_indices = 
    Angular_momentum.zkey_array
      (Angular_momentum.Quartet
         Contracted_shell.(totAngMom shell_a, totAngMom shell_b,
                           totAngMom shell_c, totAngMom shell_d))
  in

  let contracted_class = 
    Array.make (Array.length class_indices) 0.;
  in

  (** Screening on the product of coefficients *)
  let coef_max_p =
    Array.fold_left (fun accu x ->
      if (abs_float x) > accu then (abs_float x) else accu)
      0. shell_p.ContractedShellPair.coef
  and coef_max_q =
    Array.fold_left (fun accu x ->
      if (abs_float x) > accu then (abs_float x) else accu)
      0. shell_q.ContractedShellPair.coef
  in

  let rec build_list cutoff vec accu = function
  | -1 -> Array.of_list accu
  | k  -> build_list cutoff vec ( 
            if (abs_float vec.(k) > cutoff) then (k::accu)
            else accu ) (k-1)
  in
  let p_list =
    let vec = shell_p.ContractedShellPair.coef  in
    build_list (cutoff /. coef_max_q) vec [] (Array.length vec - 1)
  and q_list =
    let vec = shell_q.ContractedShellPair.coef  in
    build_list (cutoff /. coef_max_p) vec [] (Array.length vec - 1)
  in

(*
  if (Array.length p_list) < (Array.length shell_p.ContractedShellPair.coef) then
    begin
      Printf.printf "Reduced p from %d to %d:\n" (Array.length
        shell_p.ContractedShellPair.coef) (Array.length p_list);
      Array.iter (fun k -> Printf.printf "%d " k) p_list; print_newline ();
      Printf.printf "\n%!" 
    end;

  if (Array.length q_list) < (Array.length shell_q.ContractedShellPair.coef) then
    begin
      Printf.printf "Reduced q from %d to %d:\n" (Array.length
        shell_q.ContractedShellPair.coef) (Array.length q_list);
      Array.iter (fun k -> Printf.printf "%d " k) q_list; print_newline ();
      Printf.printf "\n%!" 
    end;
    *)

  let np, nq = 
    Array.length p_list,
    Array.length q_list
  in
  let filter_p vec = Array.init np (fun k -> vec.(p_list.(k)))
  and filter_q vec = Array.init nq (fun k -> vec.(q_list.(k)))
  in
  let sp = filter_p sp
  and sq = filter_q sq
  in


  (* Compute all integrals in the shell for each pair of significant shell pairs *)

  begin
    match Contracted_shell.(totAngMom shell_a, totAngMom shell_b,
                            totAngMom shell_c, totAngMom shell_d) with
    | Angular_momentum.(S,S,S,S) ->
        contracted_class.(0) <-
          let expo_inv_p = 
            Vec.init np (fun ab -> sp.(ab-1).ShellPair.expo_inv) 
          and expo_inv_q = 
            Vec.init nq (fun cd -> sq.(cd-1).ShellPair.expo_inv) 
          in

          let coef =
            let result = Mat.make0 nq np in
            Lacaml.D.ger
              (Vec.of_array @@ filter_q shell_q.ContractedShellPair.coef)
              (Vec.of_array @@ filter_p shell_p.ContractedShellPair.coef)
              result;
            result
          in
          let zm_array = Mat.init_cols np nq (fun i j ->
                  try
                    if (abs_float coef.{j,i} ) < 1.e-3*.cutoff then
                      raise NullQuartet;

                    let expo_pq_inv =
                      expo_inv_p.{i} +. expo_inv_q.{j}
                    in
                    let center_pq =
                      Coordinate.(sp.(i-1).ShellPair.center |- sq.(j-1).ShellPair.center)
                    in
                    let norm_pq_sq =
                      Coordinate.dot center_pq center_pq
                    in

                    let zero_m_array = 
                      zero_m ~maxm:0 ~expo_pq_inv ~norm_pq_sq
                    in
                    zero_m_array.(0)
                  with NullQuartet -> 0.
          ) in
          Mat.gemm_trace zm_array coef
    | _ -> 

      let coef =
        let cp = filter_p shell_p.ContractedShellPair.coef
        and cq = filter_q shell_q.ContractedShellPair.coef
        in
        Array.init np (fun l -> Array.init nq (fun k -> cq.(k) *. cp.(l)) )
      in

      let expo_inv_p = 
        Array.map (fun shell_ab -> shell_ab.ShellPair.expo_inv) sp
      and expo_inv_q = 
        Array.map (fun shell_cd -> shell_cd.ShellPair.expo_inv) sq 
      in

      let expo_b =
        Array.map (fun shell_ab -> Contracted_shell.expo shell_b shell_ab.ShellPair.j) sp
      and expo_d =
        Array.map (fun shell_cd -> Contracted_shell.expo shell_d shell_cd.ShellPair.j) sq
      in
      let norm_coef_scale_p = shell_p.ContractedShellPair.norm_coef_scale in

      let center_pq =
        Array.init 3 (fun xyz ->
          Array.init np (fun ab ->
            let shell_ab = sp.(ab) in
            Array.init nq (fun cd ->
                let shell_cd = sq.(cd)
                in
                let cpq = 
                  Coordinate.(shell_ab.ShellPair.center |- shell_cd.ShellPair.center)
                in
                match xyz with
                | 0 -> Coordinate.x cpq;
                | 1 -> Coordinate.y cpq;
                | 2 -> Coordinate.z cpq;
                | _ -> assert false
              ) 
            )
          )
      in
      let zero_m_array =
        let result = 
          Array.init (maxm+1) (fun _ ->
            Array.init np (fun _ -> Array.make nq 0. ) )
        in
        let empty = Array.make (maxm+1) 0. in
        Array.iteri (fun ab shell_ab ->
            let zero_m_array_tmp =
              Array.mapi (fun cd shell_cd ->
                  if (abs_float coef.(ab).(cd) < cutoff) then
                    empty
                  else
                    let expo_pq_inv =
                      expo_inv_p.(ab) +. expo_inv_q.(cd)
                    in
                    let norm_pq_sq =
                      center_pq.(0).(ab).(cd) *. center_pq.(0).(ab).(cd) +.
                      center_pq.(1).(ab).(cd) *. center_pq.(1).(ab).(cd) +.
                      center_pq.(2).(ab).(cd) *. center_pq.(2).(ab).(cd) 
                    in

                    zero_m ~maxm ~expo_pq_inv ~norm_pq_sq
              ) sq
            in
            (* Transpose result *)
              for m=0 to maxm do
                for cd=0 to nq-1 do
                  result.(m).(ab).(cd) <- zero_m_array_tmp.(cd).(m) *. coef.(ab).(cd)
                done
              done
          ) sp;
          result
        in

        let norm = 
          let norm_coef_scale_q = shell_q.ContractedShellPair.norm_coef_scale in
          Array.map (fun v1 ->
              Array.map (fun v2 -> v1 *. v2) norm_coef_scale_q
            ) norm_coef_scale_p
          |> Array.to_list
          |> Array.concat
        in

        let map_1d = Array.init (maxm+1) (fun _ -> Zmap.create (4*maxm))
        and map_2d = Array.init (maxm+1) (fun _ -> Zmap.create (Array.length class_indices))
        in
          (* Compute the integral class from the primitive shell quartet *)
            Array.iteri (fun i key ->
                let a = Zkey.to_int_array Zkey.Kind_12 key in
                let (angMomA,angMomB,angMomC,angMomD) =
                  ( [| a.(0) ;  a.(1) ;  a.(2) |], 
                    [| a.(3) ;  a.(4) ;  a.(5) |], 
                    [| a.(6) ;  a.(7) ;  a.(8) |], 
                    [| a.(9) ; a.(10) ; a.(11) |] )
                in
                let integral = 
                  hvrr_two_e_vector (angMomA, angMomB, angMomC, angMomD)
                    (Contracted_shell.totAngMom shell_a, Contracted_shell.totAngMom shell_b,
                    Contracted_shell.totAngMom shell_c, Contracted_shell.totAngMom shell_d)
                    (maxm, zero_m_array)
                    (expo_b, expo_d)
                    (expo_inv_p, expo_inv_q)
                    (shell_p.ContractedShellPair.center_ab,
                     shell_q.ContractedShellPair.center_ab, center_pq)
                     map_1d map_2d np nq
                in
                contracted_class.(i) <- contracted_class.(i) +. integral *. norm.(i)
              ) class_indices

  end;

  let result = 
    Zmap.create (Array.length contracted_class)
  in
  Array.iteri (fun i key -> Zmap.add result key contracted_class.(i)) class_indices;
  result



(** Computes all the two-electron integrals of the contracted shell quartet *)
let contracted_class ~zero_m shell_a shell_b shell_c shell_d : float Zmap.t =

  let shell_p = ContractedShellPair.create ~cutoff shell_a shell_b
  and shell_q = ContractedShellPair.create ~cutoff shell_c shell_d
  in
  contracted_class_shell_pairs ~zero_m shell_p shell_q