Rapid Production of Accurate Embedded-Atom Method Potentials for Metal Alloys

A critical limitation to the wide-scale use of classical molecular dynamics for alloy design is the limited availability of suitable interatomic potentials. Here, we introduce the Rapid Alloy Method for Producing Accurate General Empirical Potentials or RAMPAGE, a computationally economical procedure to generate binary embedded-atom model potentials from already-existing single-element potentials that can be further combined into multi-component alloy potentials. We present the quality of RAMPAGE calibrated Finnis-Sinclair type EAM potentials using binary Ag-Al and ternary Ag-Au-Cu as case studies. We demonstrate that RAMPAGE potentials can reproduce bulk properties and forces with greater accuracy than that of other alloy potentials. In some simulations, it is observed the quality of the optimized cross interactions can exceed that of the original off-the-shelf elemental potential inputs

Dynamics in entangled polyethylene melts

Polymer dynamics creates distinctive viscoelastic behavior as a result of a coupled interplay of motion at the atomic length scale and motion of the entire macromolecule. Capturing the broad time and length scales of polymeric motion however, remains a challenge. Using linear polyethylene as a model system, we probe the effects of the degree of coarse graining on polymer dynamics. Coarse-grained (CG) potentials are derived using iterative Boltzmann inversion with λ methylene groups per CG bead (denoted CGλ) with λ = 2,3,4 and 6 from a fully-atomistic polyethylene melt simulation. By rescaling time in the CG models by a factor α, the chain mobility for the atomistic and CG models match. We show that independent of the degree of coarse graining, all measured static and dynamic properties are essentially the same once the dynamic scaling factor α and a non-crossing constraint for the CG6 model are included. The speedup of the CG4 model is about 3 times that of the CG3 model and is comparable to that of the CG6 model. Using these CG models we were able to reach times of over 500 μs, allowing us to measure a number of quantities, including the stress relaxation function, plateau modulus and shear viscosity, and compare directly to experiment

Dynamics in entangled polyethylene melts

Polymer dynamics creates distinctive viscoelastic behavior as a result of a coupled interplay of motion at the atomic length scale and motion of the entire macromolecule. Capturing the broad time and length scales of polymeric motion however, remains a challenge. Using linear polyethylene as a model system, we probe the effects of the degree of coarse graining on polymer dynamics. Coarse-grained (CG) potentials are derived using iterative Boltzmann inversion with λ methylene groups per CG bead (denoted CGλ) with λ = 2,3,4 and 6 from a fully-atomistic polyethylene melt simulation. By rescaling time in the CG models by a factor α, the chain mobility for the atomistic and CG models match. We show that independent of the degree of coarse graining, all measured static and dynamic properties are essentially the same once the dynamic scaling factor α and a non-crossing constraint for the CG6 model are included. The speedup of the CG4 model is about 3 times that of the CG3 model and is comparable to that of the CG6 model. Using these CG models we were able to reach times of over 500 μs, allowing us to measure a number of quantities, including the stress relaxation function, plateau modulus and shear viscosity, and compare directly to experiment

Phases in the (1-x)Na0.5Bi0.5TiO3-(x)CaTiO3 system

The structures of (1 - x) Na0.5Bi0.5TiO3-(x) CaTiO3 at room temperature have been investigated using neutron powder diffraction and dielectric studies. The system exhibits an orthorhombic (Pbnm) structure for x >= 0.15 and rhombohedral (R3c) for x <= 0.05. For x = 0.10, though the neutron diffraction pattern shows features of the orthorhombic (Pbnm) structure, Rietveld refinement using this structure shows a drastic reduction in the in-phase tilt angle (similar to 4 degrees) as compared to the corresponding value (similar to 8 degrees) for a neighbouring composition x = 0.15. The neutron diffraction pattern of x = 0.10 could be fitted equally well using a two-phase model (R3c + Pbnm) with orthorhombic as the minor phase (22%), without the need for a drastic decrease in the in-phase tilt angle. The dielectric studies of x = 0.10 revealed the presence of the polar R3c phase, thereby favouring the phase coexistence model, instead of a single-phase Pbnm structure, for this composition

Cytochrome P450 (CYP2C9FNx012,FNx013) & vitamin-K epoxide reductase complex (VKORC1 -1639G < A) gene polymorphisms & their effect on acenocoumarol dose in patients with mechanical heart valve replacement

Background & objectives: Studies have demonstrated the effect of CYP2C9 (cytochrome P450) and VKORC1 (vitamin K epoxide reductase complex) gene polymorphisms on the dose of acenocoumarol. The data from India about these gene polymorphisms and their effects on acenocoumarol dose are scarce. The aim of this study was to determine the occurrence of CYP2C9FNx012,FNx013 and VKORC 1 -1639G>A gene polymorphisms and to study their effects on the dose of acenocoumarol required to maintain a target International Normalized Ratio (INR) in patients with mechanical heart valve replacement. Methods: Patients from the anticoagulation clinic of a tertiary care hospital in north India were studied. The anticoagulation profile, INR (International Normalized Ratio) values and administered acenocoumarol dose were obtained from the clinical records of patients. Determination of the CYP2C9FNx012,FNx013 and VKORC1 -1639G>A genotypes was done by PCR-RFLP (restriction fragment length polymorphism). Results: A total of 111 patients were studied. The genotype frequencies of CYP2C9 FNx011/FNx011,FNx011/FNx012,FNx011/FNx013 were as 0.883, 0.072, 0.036 and that of VKORC1 -1639G>A for GG, AG, and AA genotypes were 0.883, 0.090, and 0.027, respectively. The percentage of patients carrying any of the variant alleles of CYP2C9 and VKORC1 in heterozygous or homozygous form was 34% among those receiving a low dose of ≤20 mg/wk while it was 13.8 per cent in those receiving >20 mg/wk (P=0.014). A tendency of lower dose requirements was seen among carriers of the studied polymorphisms. There was considerable variability in the dose requirements of patients with and without variant alleles. Interpretation & conclusions: The study findings point towards the role of CYP2C9 and VKORC1 gene polymorphisms in determining the inter-individual dose variability of acenocoumarol in the Indian patients with mechanical heart valve replacement