A six-dimensional analytic potential energy surface for NH3 has been obtained using ab initio CCSD(T) data. The variational method has been used to determine its vibrational energy levels. The parameters in the surface which describe inversion have been refined such that the vibrational levels for inversion give best agreement with observation. Nearly all the fundamentals and overtones of the refined surface then agree with observation to within 1 cm -1. The barrier to inversion is found to be 1769 cm- 1. This is compared with other computed estimates of 1766 and 1777 cm -1 which appear in the literature.