We use ab initio simulations to investigate the properties of a sodium borosilicate glass of composition 3Na2 O-B2 O3 -6SiO2 . We find that the broadening of the first peak in the radial distribution functions gBO (r) and gBNa (r) is due to the presence of trigonal and tetrahedral boron units as well as to nonbridging oxygen atoms connected to BO3 units. In agreement with experimental results, we find that the [3] B units involve a significant number of nonbridging oxygens, whereas the vast majority of [4] B have only bridging oxygens. We determine the three-dimensional distribution of the Na atoms around the [3] B and [4] B units and use this information to explain why the sodium atoms associated with the latter share more oxygen atoms with the central boron atoms than the former units. From the distribution of the electrons we calculate the total electronic density of states, as well its decomposition into angular momentum contributions. The vibrational density of states shows at high frequencies a band that originates from the motion of the boron atoms. We find that the [3] B and [4] B units give rise to well-defined features in the spectrum, which thus can be used to estimate the concentration of these structural entities. The contribution of [3] B can be decomposed further into symmetric and asymmetric parts that can also be easily identified in the spectrum. Furthermore, it is found that certain features in the spectrum can be used to obtain information on the type of atom that is the second-nearest neighbor of a boron in the [4] B unit. We calculate the average Born charges on the bridging and nonbridging oxygen atoms and show that these depend linearly on the angle between the two bonds and the distance from the connected cation, respectively. Finally, we have determined the frequency dependence of the dielectric function, as well as the absorption spectra. The latter is in good quantitative agreement with the experimental data.

We use ab initio simulations to study the static and dynamic properties of a sodium borosilicate liquid with composition 3Na2 O–B2 O3 –6SiO2 , i.e., a system that is the basis of many glass-forming materials. In particular, we focus on the question how boron is embedded into the local structure of the silicate network liquid. From the partial structure factors we conclude that there is a weak nanoscale phase separation between silicon and boron and that the sodium atoms form channel-like structures as they have been found in previous studies of sodosilicate glass-formers. Our results for the x-ray and neutron structure factor show that this feature is basically not detectable in the former but should be visible in the latter as a small peak at small wave vectors. At high temperatures we find a high concentration of threefold coordinated boron atoms which decreases rapidly with decreasing T , whereas the number of fourfold coordinated boron atoms increases. Therefore, we conclude that at the experimental glass transition temperature most boron atoms will be fourfold coordinated. We show that the transformation of [3] B into [4] B with decreasing T is not just related to the diminution of nonbridging oxygen atoms as claimed in previous studies, but to a restructuring of the silicate matrix. The diffusion constants of the various elements show an Arrhenius behavior and we find that the one for boron has the same value as the one of oxygen and is significantly larger than the one of silicon. This shows that these two network-formers have rather different dynamical properties, a result that is also confirmed from the time dependence of the van Hove functions. Finally, we show that the coherent intermediate scattering function for the sodium atoms is very different from the incoherent one and that it tracks the one of the matrix atoms.

The vibrational properties of silica glass have been intensively studied experimentally and theoretically during the last four decades. However there are few theoretical studies of the evolution of the vibrational properties of more complex silicate glasses (binary, ternary etc...).We have used first principles simulations in order to investigate the properties of two silicate glasses: a sodium silicate of composition 4Na2O-6SiO2 (NS1.5) and a sodium borosilicate of composition 3Na2O-B2O3-6SiO2 (NBS). The studies were carried up using first principles molecular dynamics within the density functional theory framework as implemented in the VASP code. We have studied the structural and vibrational properties of these two glasses, and we have identified  the contributions of the various species to the vibrational density of states (VDOS) as well as to the IR spectra.In particular we will discuss how the presences of the sodium and boron atoms affect the relevant vibrational parameters: positions, shapes and intensities of the main peaks of the VDOS and IR spectra. We have found that 3- and 4-fold coordinated boron atoms give rise to distinguished spectral features. Moreover, the partial vibrational density of the 3-fold coordinated B atoms has been found to be a weighted sum of 2 specific contributions so-called 3-fold symmetric coordinated B atoms and asymmetric coordinated B atoms.

The art of deriving classical potential consists in finding the correct balance between the complexity of the functional form whose implementation should be as simple as possible, the computational efficiency, and the reliabilty. In this talk, we will present a new methodology which uses ab initio data for parametrizing classical potentials suited for amorphous systems.We have considered a simple and widespread empirical Ansatz, i.e. Born-Mayer-Coulomb, with the goal to investigate large time/size scales. As a first condition, a classical potential should be able to describe the atomic structure of the system under investigation. In order to optimize the parameters of the classical potential we have then used a cost function that involves the partial pair correlation functions extracted from ab initio simulations. The system considered is liquid silica, a liquid which represents a prototype of strong glass formers exibiting a network structure. From a methodological point of view, we will show, through the different comparisons between ab initio and classical MD, that this structural fitting procedure is a very promising approach to obtain new effective potentials suited for describing not only amorphous material but also crystalline properties.We will present simulations carried out using this new potential and showing that CHIK reproduces with more accuracy a number of properties of molten and glass silica, than the well known BKS potential. Some tests performed on a-quartz also showed that the new potential can also be used to simulate crystalline systems with a good accuracy regarding properties such as asymmetric coordinates and elastic constants. Finally, we will discuss molecular dynamics simulations carried out to represent the structures of amorphous dehydroxylated silica surface in liquid and glass states, and show that the new potential gives a also fair description of the surface.