NH3 Inversion Doubling

 I would like to add a segment to a physical chemistry lab on the infrared
 spectrum of ammonia in which students calculate an ab initio potential for the
 umbrella mode, and then calculate the infrared spectrum of the molecule from
 the potential (semi-empirical would be ok).  What is the lowest level of
 theory (i.e. the cheapest), which will yield reasonable numbers (i.e., within
 10%) for NH3 normal modes, and for the inversion doubling.
 					Sam Abrash
 					Abrash # - at - # URVAX.URiCh.EDU