Infrared transmission measurements on the S = 1 antiferromagnetic chain compound NENP in applied magnetic fields show a sharp absorption line at the field-shifted Haldane gap. This violates a wave-vector selection rule of the Hamiltonian normally used for NENP, as the gap excitations occur at the Brillouin zone boundary. We argue that the crystal structure admits terms which can explain the absorption lines. In addition, in an applied field, staggered orientations of the g-tensors produce a staggered magnetic field. This can explain the observation of a finite gap at all applied fields. Typeset using REVTEX 1 The one dimensional S = 1 antiferromagnet NENP has provided detailed experimental evidence -  for the current theoretical picture -  of S = 1 antiferromagnetic chains, and continues to be the subject of a number of investigations. However, there remain some puzzling experimental results. One of these involve
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