type units = 
| Bohr
| Angstrom

type angle_units =
| Degree
| Radian

let pi = acos (-1.)

let to_degree x =
  assert (x <= 2.*.pi);
  assert (x >= -2.*.pi);
  x *. 180. /. pi 

let to_radian x =
  assert (x <= 360.);
  assert (x >= -360.);
  x *. pi /. 180.

let angstrom_to_bohr = 1. /. 0.52917721092
let bohr_to_angstrom = 0.52917721092
;;