let bohr_to_angstrom { Bohr.x ; y ; z } = 
  let b_to_a b = Constants.a0 *. b in
  Angstrom.make 
  {
    Point.
    x = b_to_a x ;
    y = b_to_a y ;
    z = b_to_a z ;
  }



let angstrom_to_bohr { Angstrom.x ; y ; z } =  
  let a_to_b a = Constants.a0_inv *. a in
  Bohr.make 
  {
    Point.
    x = a_to_b x ;
    y = a_to_b y ;
    z = a_to_b z ;
  }