Covalent character of hydrogen bonds II

I finally read through the paper, Covalency of the Hydrogen Bond in Ice: A Direct X-Ray Measurement by E. D. Isaacs, A. Shukla, P. M. Platzman, D. R. Hamann, B. Barbiellini, and C. A. Tulk.

They do a Compton scattering experiment (X-rays are inelastically scattered of the electrons) on an ice crystal. The paragraph below explains the basic physics.

The key figure in the paper is below. The red dots show the measured difference between the momentum dependence of the Compton scattering in different directions. The solid curve is the prediction of a band structure calculation  which implicitly assumes complete quantum coherence (i.e. covalency). In contrast an electrostatic model (with no quantum coherence) gives the dot-dashed line which exhibits little anisotropy. The peaks in the inset at 1.7 and 2.85 Angstroms correspond to the H bond length and shortest Oxygen-Oxygen distance.

Aside. With regard to the solid curve above, the authors state, "There are no adjustable parameters in the theory except that a 40% reduction of the theory is required". Sounds like one adjustable parameter, to me!