Concentration fluctuations on intermediate range distances in liquid GeSe2: the critical role of ionicity

In the attempt to identify the physical origin of a first sharp diffraction peak (FSDP) in the concentration-concentration partial structure factor, we perform first-principle molecular dynamics of liquid GeSe2. We compare the structural properties of two set of simulations, which differ by their treatment of the electronic structure. This is achieved by considering different energy cutoffs defining the plane-wave basis sets, the higher energy cutoff leading to a moderate enhancement of the ionicity in the system. Both schemes give a neutron scattering factor in excellent agreement with experiment. The higher ionicity leads to small but perceptible modifications in some structural features related to short and intermediate range order, slightly improving the agreement with the experimental partial structure factors. In particular, a FSDP becomes discernable in the concentration-concentration structure factor. Although the magnitude of this peak is small compared to the experiment, this observation suggests that a good description of this peak could be obtained with an energy functional inducing an even higher ionicity. (C) 2000 Elsevier Science B.V. All rights reserved.