Comment on Tuning of the CDW in MX Chains
  In M. Alouani et al., Phys. Rev. Lett. 71, 1415 (1993), the authors performed local-density-approximation (LDA) calculations and fit to the two-band model [J.T. Gammel et al., Phys. Rev. B 45, 6408 (1992)] we proposed for halogen-bridged transition-metal linear-chain compounds ($M$$X$ chains). Based on that fit, the authors argue a hard-core repulsive term must be added to the lattice potential in our model. However, the authors fit only to the variation in total energy as a function of percent dimerization for a fixed $M$$M$ distance and did not fit the variation in total energy with $M$$M$ distance. Thus the fit was incorrectly calculated, the LDA total energy surface is {\it not} reproduced by the parameters listed in this paper, and the conclusion as to the importance of anharmonic corrections must be reexamined. In contradiction to their conclusion based on this erroneous fit, I show here that the harmonic approximation (linear $M$$X$ springs to model the lattice energy) is in fact very good, and a small quartic correction yields quantitative agreement with the LDA total energy, though to this order more realistic electron-phonon interactions should be included in the model, as discussed in I. Batistic et al., Phys. Rev. B 48, 6065 (1993). 1992 PACS: 71.38.+i, 71.25.Tn, 71.20.Hk 
J. Tinka Gammel