  We derive direct single-stage numerical evaluation of the electronic overlap
integral between arbitrary atomic orbitals (including STOs). Integration is
over cartesian co-ordinates, and replaces previous sums over 'special'
functions. The results, in Mathematica 10 and Maple 18, agree with the
literature to 8 digits. We briefly discuss possible use in quantum chemistry,
including accuracy, algorithmic suitability and operating-system
machine-implementation as an intrinsic function.
