(** Atomic shell pair *)

(** Data structure to represent pairs of atomic shells. The products of
  * functions in the shell pair are one-electron functions.
  * 
  * An atomic shell pair is an array of pairs of contracted shells.
  *)


(** Type *)

type t

open Common

(** Access *)

val atomic_shell_a         : t -> Atomic_shell.t
(** Returns the first  ~Atomic_shell.t~ which was used to build the atomic
  * shell pair. *)

val atomic_shell_b         : t -> Atomic_shell.t
(** Returns the second ~Atomic_shell.t~ which was used to build the atomic
  * shell pair. *)

val contracted_shell_pairs : t -> Contracted_shell_pair.t list
(** Returns an array of ~ContractedShellPair.t~, containing all the pairs of
  * contracted functions used to build the atomic shell pair.
  *)

val ang_mom                : t -> Angular_momentum.t
(** Total angular Momentum *)

val monocentric            : t -> bool
(** If true, the two atomic shells have the same center. *)

val norm_scales            : t -> float array
(** norm_coef.(i) / norm_coef.(0) *)

val a_minus_b              : t -> Coordinate.t
(** Returns $A-B$ *)

val a_minus_b_sq           : t -> float
(** Returns $\vert A-B \vert^2$ *)


(** Creation *)

val make : ?cutoff:float -> Atomic_shell.t -> Atomic_shell.t -> t option
(* Creates an atomic shell pair from two atomic shells.
 * 
 * The contracted shell pairs contains the only pairs of primitives for which
 * the norm is greater than ~cutoff~. 
 * 
 * If all the contracted shell pairs are not significant, the function returns
 * ~None~. *)

val of_atomic_shell_array : ?cutoff:float -> Atomic_shell.t array -> t option array array
(** Creates all possible atomic shell pairs from an array of atomic shells.
  * If an atomic shell pair is not significant, sets the value to ~None~.
  *)



(** Printers *)

val pp : Format.formatter -> t -> unit