Breaking of the overall permutation symmetry in nonlinear optical   susceptibilities of one-dimensional periodic dimerized Huckel model
  Based on infinite one-dimensional single-electron periodic models of trans-polyacetylene, we show analytically that the overall permutation symmetry of nonlinear optical susceptibilities is, albeit preserved in the molecular systems with only bound states, no longer generally held for the periodic systems. The overall permutation symmetry breakdown provides a fairly natural explanation to the widely observed large deviations of Kleinman symmetry for periodic systems in off-resonant regions. Physical conditions to experimentally test the overall permutation symmetry break are discussed. 
Minzhong Xu, Shidong Jiang