Revisiting Quantum discord for two-qubit X states: Error bound to   Analytical formula
  In this article, we investigate the error bound of quantum discord, obtained by the analytic formula of Ali et al.[Phys. Rev. A 81(2010), 042105] in case of general X states and by the analytic formula of Fanchini et al.[Phys. Rev. A 81(2010), 052107] in case of symmetric X states. We show that results of Ali et. al. to general X states and Fanchini et al. to symmetric X states may have worst-case error of 0.004565 and 0.0009 respectively. 
Min Namkung, Jinho Chang, Jaehee Shin and Younghun Kwon
quant-ph