(** Number of alpha and beta electrons *)

type t = {
  n_alpha : int ;
  n_beta  : int ;
  multiplicity : int;
}


let make ?multiplicity:(multiplicity=1) ?charge:(charge=0) nuclei =
  let positive_charges =
    Array.fold_left (fun accu (e, _) -> accu + Charge.to_int (Element.to_charge e) )
      0 nuclei
  in
  let negative_charges = charge - positive_charges in
  let n_elec = - negative_charges in
  let n_beta = ((n_elec - multiplicity)+1)/2 in
  let n_alpha = n_elec - n_beta in
  let result = { n_alpha ; n_beta ; multiplicity } in
  if multiplicity <> (n_alpha - n_beta)+1 then
    invalid_arg (__FILE__^": make");
  result

let charge e =
  - (e.n_alpha + e.n_beta)
  |> Charge.of_int