Truncated Sum Rules and their use in Calculating Fundamental Limits of   Nonlinear Susceptibilities
  Truncated sum rules have been used to calculate the fundamental limits of the nonlinear susceptibilities; and, the results have been consistent with all measured molecules. However, given that finite-state models result in inconsistencies in the sum rules, it is not clear why the method works. In this paper, the assumptions inherent in the truncation process are discussed and arguments based on physical grounds are presented in support of using truncated sum rules in calculating fundamental limits. The clipped harmonic oscillator is used as an illustration of how the validity of truncation can be tested; and, several limiting cases are discussed as examples of the nuances inherent in the method. 
Mark G. Kuzyk