Neutron diffraction study of the Ti1-xMoxC alloy

In this work, the crystal structure and (RMSD) root-mean-square displacement of atoms in new cubic refractory interstitial alloys Ti0.74Mo0.26C and Ti0.70Mo0.30C were determined by neutron diffraction. These alloys are obtained by powder metallurgy by sintering fine powders of cubic titanium carbides TiC and molybdenum MoC. The values of the RMSD of atoms in these alloys, obtained by the methods of least squares and full-profile analysis from the neutron diffraction data, turned out to be identical. They turned out to be much larger than in the stoichiometric TiC alloy, which has a similar cubic structure and close geometric parameters of atoms with the Ti0.74Mo0.26C and Ti0.70Mo0.30C alloys. These alloys were explained by the occurrence of large static distortions in the lattice of the Ti0.70Mo0.30C alloy due to the differences in the masses and atomic radii of the titanium and molybdenum elements.

The effect of cooling rate on structure and mechanical properties of Co-Cr-Cu-Fe-Ni-Sn high entropy alloys

The paper examines the structure and mechanical properties of multicomponent high-entropy CoCrCuFeNiSnx alloys in as-cast and splat-quenched states. The compositions of investigated alloys is analyzed by using the criteria for predicting the phase composition of high-entropy alloys available in the literature, based both on calculations of the entropy and enthalpy of mixing and on an estimate of the difference between the atomic radii of the component. The alloy films is fabricated by the known technique of splat-quenching. A cooling rate estimated by a film thickness is ~ 106 K/s. The simultaneous formation of two solid solutions (FCC + ordered BCC) has been established in the alloy structure. An increase in the concentration of Sn favors the formation of the ordered (В2 type) solid solution in the structure of the alloys. High values of the microhardness and dislocation density have been obtained in the splat-quenched samples. It is also shown that an increase in the Sn content positively affects the microhardness.

Optimized Halogen Atomic Radii for PBSA Calculations Using Off-Center Point-Charges

<div>In force field methods, the usage of off-center point-charges, also called extra-points (EPs), is a common strategy to tackle the anisotropy of the electrostatic potential of covalently-bonded halogens (X), thus allowing the description of halogen bonds (XBs) at the molecular mechanics / molecular dynamics (MM/MD) level. Diverse EP implementations exist in the literature differing on the charge sets and/or the X–EP distances. Poisson–Boltzmann and surface area (PBSA) calculations can be used to obtain solvation free energies (∆G solv ) of small molecules, often to compute binding free energies (∆G bind ) at the MM PBSA level. This method depends, among other parameters, on the empirical assignment of atomic radii (PB radii). Given the multiplicity of off-center point-charges models and the lack of specific PB radii for halogens compatible with such implementations, in this work we assessed the performance of PBSA calculations for the estimation of ∆G solv values in water (∆G hyd ), also conducting an optimization of the halogen PB radii (Cl, Br, and I) for each EP model. We not only expand the usage of EP models in the scope of the General AMBER Force Field (GAFF) but also provide the first optimized halogen PB radii in the context of the CHARMM General Force Field (CGenFF), thus contributing to improving the description of halogenated compounds in PBSA calculations.</div>

Optimized Atomic Partial Charges and Radii Defined by Radical Voronoi Tessellation of Bulk Phase Simulations

We present a novel method for the computation of well-defined optimized atomic partial charges and radii from the total electron density. Our method is based on a two-step radical Voronoi tessellation of the (possibly periodic) system and subsequent integration of the total electron density within each Voronoi cell. First, the total electron density is partitioned into the contributions of each molecule, and subsequently the electron density within each molecule is assigned to the individual atoms using a second set of atomic radii for the radical Voronoi tessellation. The radii are optimized on-the-fly to minimize the fluctuation (variance) of molecular and atomic charges. Therefore, our method is completely free of empirical parameters. As a by-product, two sets of optimized atomic radii are produced in each run, which take into account many specific properties of the system investigated. The application of an on-the-fly interpolation scheme reduces discretization noise in the Voronoi integration. The approach is particularly well suited for the calculation of partial charges in periodic bulk phase systems. We apply the method to five exemplary liquid phase simulations and show how the optimized charges can help to understand the interactions in the systems. Well-known effects such as reduced ion charges below unity in ionic liquid systems are correctly predicted without any tuning, empiricism, or rescaling. We show that the basis set dependence of our method is very small. Only the total electron density is evaluated, and thus, the approach can be combined with any electronic structure method that provides volumetric total electron densities—it is not limited to Hartree–Fock or density functional theory (DFT). We have implemented the method into our open-source software tool TRAVIS.

Optimized Halogen Atomic Radii for PBSA Calculations Using Off-Center Point-Charges

<div>In force field methods, the usage of off-center point-charges, also called extra-points (EPs), is a common strategy to tackle the anisotropy of the electrostatic potential of covalently-bonded halogens (X), thus allowing the description of halogen bonds (XBs) at the molecular mechanics / molecular dynamics (MM/MD) level. Diverse EP implementations exist in the literature differing on the charge sets and/or the X–EP distances. Poisson–Boltzmann and surface area (PBSA) calculations can be used to obtain solvation free energies (∆G solv ) of small molecules, often to compute binding free energies (∆G bind ) at the MM PBSA level. This method depends, among other parameters, on the empirical assignment of atomic radii (PB radii). Given the multiplicity of off-center point-charges models and the lack of specific PB radii for halogens compatible with such implementations, in this work we assessed the performance of PBSA calculations for the estimation of ∆G solv values in water (∆G hyd ), also conducting an optimization of the halogen PB radii (Cl, Br, and I) for each EP model. We not only expand the usage of EP models in the scope of the General AMBER Force Field (GAFF) but also provide the first optimized halogen PB radii in the context of the CHARMM General Force Field (CGenFF), thus contributing to improving the description of halogenated compounds in PBSA calculations.</div>

Optimized Halogen Atomic Radii for PBSA Calculations Using Off-Center Point-Charges

<div>In force field methods, the usage of off-center point-charges, also called extra-points (EPs), is a common strategy to tackle the anisotropy of the electrostatic potential of covalently-bonded halogens (X), thus allowing the description of halogen bonds (XBs) at the molecular mechanics / molecular dynamics (MM/MD) level. Diverse EP implementations exist in the literature differing on the charge sets and/or the X–EP distances. Poisson–Boltzmann and surface area (PBSA) calculations can be used to obtain solvation free energies (∆G solv ) of small molecules, often to compute binding free energies (∆G bind ) at the MM PBSA level. This method depends, among other parameters, on the empirical assignment of atomic radii (PB radii). Given the multiplicity of off-center point-charges models and the lack of specific PB radii for halogens compatible with such implementations, in this work we assessed the performance of PBSA calculations for the estimation of ∆G solv values in water (∆G hyd ), also conducting an optimization of the halogen PB radii (Cl, Br, and I) for each EP model. We not only expand the usage of EP models in the scope of the General AMBER Force Field (GAFF) but also provide the first optimized halogen PB radii in the context of the CHARMM General Force Field (CGenFF), thus contributing to improving the description of halogenated compounds in PBSA calculations.</div>

Theoretical atomic radii of elements (H-Cm): A non-relativistic study with Gaussian basis set using HF, post-HF and DFT methods

We calculated the most probable radius of an atom for elements from H to Cm. The calculations were carried out by using non-relativistic, spin polarized, HF, MP2 and DFT methods with all electron Gaussian basis set. Periodicity of atomic radii was correlated with the experimental first ionization energies. This non-relativistic atomic radii were also compared with other theoretical atomic radii. With respect to the Dirac-Slater data, our values were in good agreement with the elements up to Sn. Relationship with van der Waals radii of noble gases was discussed. Anomalous properties of Gd and Pd were examined. Linearity of lanthanide contraction of elements with 4f electrons is illustrated. This linearity is found independent of the extent of electron correlation.

Theoretical Atomic Radii of Elements (H-Cm): A Non-Relativistic Study with Gaussian Basis Set Using HF, Post-HF and DFT Methods

<div><p>We calculated the most probable radius of an atom for elements from H to Cm. The calculations were carried out by using non-relativistic, spin polarized, HF, MP2 and DFT methods with all electron Gaussian basis set<i>. </i>Periodicity of atomic radii was correlated with the experimental first ionization energies. This non-relativistic atomic radii were also compared with other theoretical atomic radii. With respect to the Dirac-Slater data, our values were in good agreement with the elements up to Sn. Relationship with van der Waals radii of noble gases was discussed. Anomalous properties of Gd and Pd were examined. Linearity of lanthanide contraction of elements with <i>4f </i>electrons is illustrated. This linearity is found independent of the extent of electron correlation. S.I. give data of calculated radii and other correlated studies (with ionization energies, another theoretical radii etc.)</p></div>