The origin of the fragile-to-strong crossover in liquid silica is characterized in terms of properties of the potential-energy landscape (PEL). Using the standard BKS model [B. W. H. van Beest, G. J. Kramer, and R. A. van Santen, Phys. Rev. Lett. 64, 1955 (1990)] of silica we observe a low-energy cutoff of the PEL. It is shown that this feature of the PEL is responsible for the occurrence of the fragile-to-strong crossover and may also explain the avoidance of the Kauzmann paradox. The number of defects, i.e., deviations from the ideal tetrahedral structure, vanishes for configurations with energies close to this cutoff. This suggests a structural reason for this cutoff.

Though the existence of two-level systems (TLS) is widely accepted to explain low temperature anomalies in many physical observables, knowledge about their properties is very rare. For silica which is one of the prototype glass-forming systems we elucidate the properties of the TLS via computer simulations by applying a systematic search algorithm. We get specific information in the configuration space, i.e. about relevant energy scales, the absolute number of TLS and electric dipole moments. Furthermore important insight about the real-space realization of the TLS can be obtained. Comparison with experimental observations is included.PACS numbers: 61.43.Fs, 63.50.+x, Most kinds of disordered solids show anomalous behavior at very low temperatures (Kelvin regime and below) as compared to their crystalline counterparts. Many of the observed features can be explained by the Standard Tunneling Model (STM) [1,2] and its generalization, which is the Soft-Potential Model [3,4]. The basic idea of the STM is to postulate the possibility of localized transitions between different configurations, i.e. adjacent minima of the potential energy landscape. Such a transition can be described by a double-well potential (DWP), characterized by an asymmetry ∆, potential height V and distance d between both configurations. From a quantum-mechanically perspective at low temperatures the system is tunneling between both configurations and the DWP is characterized by the lowest two eigenstates. If their energy difference E is in the Kelvin regime, these DWP may contribute to the low-temperature anomalies. Then one may speak of Two Level Systems (TLS). The TLS can couple to strain and electric fields and therefore show up in observables like thermal conductivity, sound absorption and dielectric response [5]. Recently, even observations about the interaction of different TLS have been reported [6,7,8].So far it has not been possible to derive a theory of the low-temperature anomalies of real glass-forming systems from first principles, except for mean-field models [9] and random first order transition theory [10]. Thus for a prototype system like SiO 2 the STM has to be basically considered as phenomenological. Important questions emerge. Computer simulations may help to shed some light on the nature of the low-temperature anomalies. In previous work on SiO 2 the trajectories, generated either by molecular dynamics [11,12] or by the activationrelaxation technique [13], have been analyzed with respect to transition events between different structures. Both approaches yield some interesting insight into the nature of relaxation processes in SiO 2 . (Q1), however, requires a systematic search procedure, and (Q2) and (Q3) a sufficiently large number of characteristic DWP and thus an efficient search method. This has not been the scope of previous simulations on SiO 2 .In recent years we have developed a set of simulation techniques which allow us to approach these questions [14,15]. (Q1) Starting from representative low-energy structures we...

A systematic density functional theory based study of hydrogen bond energies of 2465 single hydrogen bonds has been performed. In order to be closer to liquid phase conditions, different from the usual reference state of individual donor and acceptor molecules in vacuum, the reference state of donors and acceptors embedded in a perfect conductor as simulated by the COSMO solvation model has been used for the calculation of the hydrogen bond energies. The relationship between vacuum and conductor reference hydrogen bond energies is investigated and interpreted in the light of different physical contributions, such as electrostatic energy and dispersion. A very good correlation of the DFT/COSMO hydrogen bond energies with conductor polarization charge densities of separated donor and acceptor atoms was found. This provides a method to predict hydrogen bond strength in solution with a root mean square error of 0.36 kcal mol À1 relative to the quantum chemical dimer calculations. The observed correlation is broadly applicable and allows for a predictive quantification of hydrogen bonding, which can be of great value in many areas of computational, medicinal and physical chemistry.

Though the existence of two-level systems (TLS) is widely accepted to explain low temperature anomalies in the sound absorption, heat capacity, thermal conductivity and other quantities, an exact description of their microscopic nature is still lacking. We performed computer simulations for a binary Lennard-Jones system, using a newly developed algorithm to locate double-well potentials (DWP) and thus two-level systems on a systematic basis. We show that the intrinsic limitations of computer simulations like finite time and finite size problems do not hamper this analysis. We discuss how the DWP are embedded in the total potential energy landscape. It turns out that most DWP are connected to the dynamics of the smaller particles and that these DWP are rather localized. However, DWP related to the larger particles are more collective.

The Conductor-Like-Screening-Model for Real Solvents (COSMO-RS) method has been used for the blind prediction of cyclohexane-water distribution coefficients logD within the SAMPL challenge. The partition coefficient logP of the neutral species was calculated first and then corrected for dissociation or protonation, as appropriate for acidic or basic solutes, to obtain the cyclohexane-water logD. Using the latest version of the COSMOtherm implementation, this approach in combination with a rigorous conformational sampling yielded a predictive accuracy of 2.11 log units (RMSD) for the 53 compounds of the blind prediction dataset. By that it was the most accurate of all contest submissions and it also achieved the best rank order. The RMSD mainly arises from a group of outliers in the negative logD range, which at least partly may arise from dimerization or other experimental problems coming up for very polar molecules in very non-polar solvents.