Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

The use of empirical force fields is now a standard approach in predicting the properties of hydrated oxides which are omnipresent in both natural and engineering applications. Transferability of force fields to analogous hydrated oxides without rigorous investigations may result in misleading property predictions. Herein, we focus on two common empirical force fields, the simple point charge ClayFF potential and the core-shell potential to study tobermorite minerals, the most prominent family of Calcium-Silicate-Hydrates that are complex hydrated oxides. We benchmark the predictive capabilities of these force fields against first principles results. While the structural information seem to be in close agreement with DFT results, we find that for higher order properties such as elastic constants, the core-shell potential quantitatively improves upon the simple point charge model, and shows a larger degree of transferability to complex materials. In return, to remedy the deficiencies of the simple point charge potential for hydrated calcio-silicates, we suggest using both structural data and elasticity data for potential calibration, a new force field potential, CSH-FF. This re-parameterized version of ClayFF is then applied to simulating an atomistic model of cement (Pellenq et al., PNAS, 2009). We demonstrate that this force field improves the predictive capabilities of ClayFF, being considerably less computational intensive than the core-shell model.

/pdf/transferability-of-empirical-force-fields-for-complex-syh4j34wg6.pdf

Transferability of empirical force fields for complex hydrated calcio-silicate layered materials

Cement-based materials are widely utilized in infrastructure. The main product of hydrated products of cement-based materials is calcium silicate hydrate (C-S-H) gels that are considered as the binding phase of cement paste. C-S-H gels in Portland cement paste account for 60–70% of hydrated products by volume, which has profound influence on the mechanical properties and durability of cement-based materials. The preparation method of C-S-H gels has been well documented, but the quality of the prepared C-S-H affects experimental results; therefore, this review studies the preparation method of C-S-H under different conditions and materials. The progress related to C-S-H microstructure is explored from the theoretical and computational point of view. The fractality of C-S-H is discussed. An evaluation of the mechanical properties of C-S-H has also been included in this review. Finally, there is a discussion of the durability of C-S-H, with special reference to the carbonization and chloride/sulfate attacks.

Structure, Fractality, Mechanics and Durability of Calcium Silicate Hydrates

Slag, an important supplementary cementitious material, is widely utilized for producing sustainable cement-based materials. In this paper, to study the effect of hydration stage and composition on the microstructure of cement-slag powder composites, the hydrated tricalcium silicate (C3S) curing at 20 °C at 28-day and 90-day with slag dosage from 0% to 45% were investigated by 29Si and 27Al NMR spectrum and molecular dynamics. Silicate connectivity from 29Si NMR test showed that with increasing slag substitution content, the mean chain length (MCL) of the C3S-slag hydrated paste at 28-day increased from 2.99 to 4.02 and the AlO4/SiO4 ratio rose from 0 to 0.043. Aluminate coordinates of 27Al NMR analysis revealed that the incorporation of slag could contribute to transformation from ettringite to Aft and formation of Al [4] in the C-A-S-H gel. Furthermore, reactive force field molecular dynamics was utilized to study the structural, reactivity and mechanical properties of C-A-S-H gel in hydrated composite. The AlO4 species in the C-A-S-H gel played an essential role in healing the defective silicate chains and transforming dimmer-rich silicate chains at low slag powder content to a long aluminate-silicate skeleton with high slag powder content. The structural evolution was attributed to the polymerization reaction between aluminate species and non-bridging silicate tetrahedron, with neighboring water molecule dissociations. In respect of dynamical properties, the aluminate-silicate structure exhibited good stability due to high Al–O–Si connection characterized by time correlated function. Uniaxial tensile modeling revealed that C-A-S-H gel with long aluminate-silicate chains exhibits good loading resistance and the cohesive strength of C-A-S-H gel is improved to a great extent due to the incorporation of slag powder. Hopefully, this study may provide molecular insights for design of sustainable and durable cement-slag powder composites.

Insights on the molecular structure evolution for tricalcium silicate and slag composite: From 29Si and 27Al NMR to molecular dynamics

Influence of substrate moisture on the interfacial bonding between calcium silicate hydrate and epoxy

The interactions between water molecules and C-S-H surfaces in loads-induced nanopores: A molecular dynamics study

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The interactions between water molecules and C-S-H surfaces in loads-induced nanopores: A molecular dynamics study