Mass-weighted intrinsic reaction coordinate calculations were carried out for the inversion/bending motion in the ground and lowest excited singlet states of NH3 and ND3. Vibrational eigenstates were obtained from these IRC potentials directly, obviating the need to make assumptions about the coordinate dependence of the reduced mass. The 0(+/-) inversion splittings for the (X) over tilde state of NH3 and ND3 are found to be 1.17 and 0.89 cm(-1), respectively, at the MP4(SDQ)/aug-cc-pVTZ level of theory. CASPT2-refined IRC scans of both the relaxed and vertical (A) over tilde -state surfaces were carried out. The (A) over tilde-(X) over tilde absorption spectra of NH3 and ND3 were modeled using the respective Boltnnann-weighted vibrational electronic transition matrix elements.