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, w e perform first-principle molecular dynamics of liquid GeSe2. We compare th e structural properties of two set of simulations, which differ by their tr eatment of the electronic structure. This is achieved by considering differ ent energy cutoffs defining the plane-wave basis sets, the higher energy cu toff leading to a moderate enhancement of the ionicity in the system. Both schemes give a neutron scattering factor in excellent agreement with experi ment. The higher ionicity leads to small but perceptible modifications in s ome structural features related to short and intermediate range order, slig htly improving the agreement with the experimental partial structure factor s. In particular, a FSDP becomes discernable in the concentration-concentra tion structure factor. Although the magnitude of this peak is small compare d to the experiment, this observation suggests that a good description of t his peak could be obtained with an energy functional inducing an even highe r ionicity. (C) 2000 Elsevier Science B.V. All rights reserved.