Silica computer simulations transition

First-Principles Simulations In the cases of carbon and silica, computer simulations using classical empirical potentials have shown a liquid-liquid transition [17,92], but first-principle MD (FPMD) simulations [93,94] show results that are not consistent with classical simulations. In silicon, Jakse and Pasturel [22] and independently Ganesh and Widom [23] have reported first-principle simulation results, both of which support the proposed liquid-liquid transition in silicon. In the work of Ganesh and Widom, the authors report the emergence of a van der Waals-like loop (shown in Fig. 13), as signature of a first-order phase transition at temperatures below 1182K. The maximum time span of these simulations is around 40 ps [22], which seems to be very small compared to the relaxation times of LDL (tens to hundreds of nanoseconds see below) obtained from simulations of SW silicon [21]. But the FPMD calculations are computationally very expensive compared to classical MD simulations. Hence, it would be of interest to compare the equilibration times of the system simulated in FPMD and classical MD and also do a systematic study of relaxation processes in these two different methods of simulation. A comparison of properties obtained in different simulations are discussed in a later section. 

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. 

Figure 5.4 Transition State Theory for diffusion in condensed media, (a) General representation of the transition state theory, (b) Diffusive jump in glassy polymer [ 17j. Reprinted from journal of Membrane Science, 73, E. Smit, M. H. V. Mulder, C. A. Smolders, H. Karrenbeld, j. van Eerden and D. Eeil, Modelling of the diffusion of carbon dioxide in polyimide matrices by computer simulation, 247 257, Copyright (1992), with permission from Elsevier, (c) Diffusive jump in microporous silica, reprinted with permission from AlChE, Theory of gas diffusion and permeation in inorganic molecular-sieve membranes by A. B. Shelekhin, A. C. Dixon and Y. H. Ma, 41, 58 67, Copyright (1995) AlChE...

In this work, we review briefly the phenomenology associated to LLPTs based on results obtained from computer simulations of different systems, such as silica, water, and atomic model systems. When possible, results from computer simulations are compared to available experiments. This work is organized as follows. In the next section, we present the phase diagram of polymorphic liquids supported by many computer simulations and experiments. We review the thermodynamics of first-order phase transitions and show how it is observed in computer Simula tions of polymorphic liquids. The relationship between liquid polymorphism and anomalous properties in liquids is also discussed. The next section also includes a description of glass polymorphism, its relation to liquid polymorphism, and a close comparison between experiments and simulations. In Section III, we describe computer simulation models of systems that present liquid polymorphism, with emphasis on the molecular interactions and common properties of these models that are thought to originate LLPTs. A summary and discussion are presented in Section IV. 

Multicomponent systems that present polyamorphism have also been reported in computer simulation studies. For example, in Ref. [35], it is found that silica has a LLCP at very low temperature. Silica is also a tetrahedral liquid and it shares many of the thermodynamic properties observed in water. In Ref. [35], two silica models were considered. In both models, the interactions among O and Si atoms are isotropic, due to single point charges and short-range interacting sites located on each atom. Both models considered in Ref. [35] are characterized by a LLCP at very low temperature and coexistence between two liquids is observed in out of equilibrium simulations close to one of the spinodal lines (see Fig. 2b). The location of the LLCP was estimated to be below the glass transition in real silica and hence, unaccessible in experiments. We note that polyamorphism in the glass state is indeed observed in compression experiments on amorphous silica [14], and is qualitatively reproduced in computer simulations [89]. Other examples of multicomponent systems that show LLPT in simulations are presented in Refs [65,90]. In these cases, a substance that already shows polymorphism is mixed with a second component. 

Saika-Voivod, I., Sciortino, R, and Poole, P.H. (2000) Computer simulations of liquid silica equation of state and liquid-liquid phase transition. Phys. Rev. E, 63,1-9. 

Liquid-vapor phase transitions of confined fluids were extensively studied both by experimental and computer simulation methods. In experiments, the phase transitions of confined fluids appear as a rapid change in the mass adsorbed along adsorption isotherms, isochores, and isobars or as heat capacity peaks, maxima in light scattering intensity, etc. (see Refs. [28, 278] for review). A sharp vapor-liquid phase transition was experimentally observed in various porous media ordered mesoporous sifica materials, which contain non-interconnected uniform cylindrical pores with radii Rp from 10 A to more than 110 A [279-287], porous glasses that contain interconnected cylindrical pores with pore radii of about 10 to 10 A [288-293], silica aerogels with disordered structure and wide distribution of pore sizes from 10 to 10" A [294-297], porous carbon [288], carbon nanotubes [298], etc. 

Maseras E, Morokuma K (1995) IMOMM a new integrated ab initio -H molecular mechanics geometry optimization scheme of equilibrium structures and transition states. J Comput Chem 16(9) 1170 Mischler C, Horbach J, Kob W, Binder K (2005) Water adsorption on amorphous silica surfaces a Car-Parrinello simulation study. J Phys Condens Matter 17(26) 4005... 

---