Effective interaction potentials for alkali and alkaline earth metal ions in SPC/E water and prediction of mean ion activity coefficients

The potential of mean force (PMF) acting between two simple ions surrounded by SPC/E water have been determined by molecular dynamics (MD) simulations using a spherical cavity approach. Such effective ion-ion potentials were obtained for Me-Me, Me-Cl-, and Cl(-)-Cl- pairs, where Me is a Li+, Na+, K+, Mg2+, Ca2+, Sr2+, and Ba2+ cation. The ionic sizes estimated from the effective potentials are not pairwise additive, a feature in the frequently used primitive model for electrolytes. The effective potentials were used in Monte Carlo (MC) simulations with implicit water to calculate mean ion activity coefficients of LiCl, NaCl, KCl, MgCl2, CaCl2, SrCl2, and BaCl2. Predicted activities were compared with experimental ones in the electrolyte concentration range 0.1-1 M. A qualitative agreement for LiCl and a satisfactory agreement for NaCl were found, whereas the predictions for KCl by two K+ models were less coherent. In the case of alkaline earth metal ions, all experimental activities were successfully reproduced at c = 0.1 M. However, at higher concentrations, similar deviations occurred for all divalent cations, suggesting that the dependence of the permittivity on the salt concentration and the polarization deficiency arising from the ordering of water molecules in the ion hydration shells are important in such systems.     

含Hg化合物的相对论赝势从头计算研究——HgX2(X=Cl,Br,I)的电子结构

本文应用相对论赝势从头计算方法, 在不同基组水平上, 系统地研究了卤化汞(HgX_2, X=Cl,Br,I)系列的电子结构。表明除Hg的6s主要参与成键外, 5dz~2也起了重要的作用。并且随卤素原子序的增加, π成键作用也增强。同时还应用单电子自旋-轨道耦合方法, 研究了旋-轨耦合效应的影响, 指定了该系列化合物的光电子能谱。     

Ab initio study including spin-orbit effects on the B-X transition of AgI

The lowest Omega = 0-,0+,1,2 fine-structure potential energy curves arising from the two lowest-lying singlet (X 1Sigma+ and 2 1Sigma+) and the first 3Pi electronic states of AgI were obtained through an effective Hamiltonian; the purely electronic LambdaSSigma energies were used as diagonal elements, which were calculated through extensive complete active space self-consistent field + averaged coupled pair functional calculations, with relativistic effective core potentials and optimized Gaussian basis sets for both atoms. The spin-orbit interactions were included using the Stuttgart effective spin-orbit potentials. For the excited Omega = 0+ states, very strong mixtures were found of the 2 1Sigma+ and 3Pi parents that lead to the fine-structure (0+) single B state (dominated by the 2 1Sigma+ parent at long distance), that explains the B <-- X transitions. The present results also explain the presence of a second long-distance minimum for the B0+ state, experimentally Rydberg-Klein-Rees fitted. These calculations produced, as a byproduct, a new lower-lying Omega = 0+ yet unobserved fine-structure state predicted to exist around 22,000 cm(-1). Our theoretical results are compared and discussed in the light of the experimental data for the B-X transitions in silver halides [J. Chem. Phys. 109, 9831 (1998)].     

Density-functional theory with effective potential expressed as a direct mapping of the external potential: applications to atomization energies and ionization potentials

In this paper the authors further develop and apply the direct-mapping density functional theory to calculations of the atomization energies and ionization potentials. Single-particle orbitals are determined by solving the Kohn-Sham [Phys. Rev. A. 140, 1133 (1965)] equations with a local effective potential expressed in terms of the external potential. A two-parametric form of the effective potential for molecules is proposed and equations for optimization of the parameters are derived using the exchange-only approximation. Orbital-dependent correlation functional is derived from the second-order perturbation theory in its Moller-Plesset-type zeroth-order approximation based on the Kohn-Sham orbitals and orbital energies. The total atomization energies and ionization potentials computed with the second-order perturbation theory were found to be in agreement with experimental values and benchmark results obtained with ab initio wave mechanics methods.     

On the possible "supersolid" character of parahydrogen clusters

We present results of a theoretical study of structural and superfluid properties of parahydrogen (p-H(2)) clusters comprising 25, 26, and 27 molecules at low temperature. The microscopic model utilized here is based on the Silvera-Goldman pair potential. Numerical results are obtained by means of quantum Monte Carlo simulations, making use of the continuous-space worm algorithm. The clusters are superfluid in the low temperature limit, but display markedly different physical behaviors. For N = 25 and 27, superfluidity at low temperature arises as clusters melt, that is, become progressively liquid-like as a result of quantum effects. On the other hand, for N = 26, the cluster remains rigid and solid-like. We argue that the cluster (p-H(2))(26) can be regarded as a mesoscopic "supersolid". This physical picture is supported by results of simulations in which a single p-H(2) molecule in the cluster is isotopically substituted.     

胶体粒子表面有效电荷的实验测量

胶体溶液中带电胶粒的有效电荷是计算粒子间相互作用势的一个重要参数. 在本文中用电导率-粒子数密度关系法和电导滴定法分别研究了七种粒径及表面带电情况均不相同的聚苯乙烯粒子, 结果显示两种测量方法得到的有效电荷数值具有较好的一致性, 误差在7%以内. 同时发现, 经验公式计算的有效电荷差不多是实验值的2倍, 表明文献中的经验公式对于本文所研究的胶体粒子体系不适用.     

Interpretation of conservative forces from Stokesian dynamic simulations of interfacial and confined colloids

This paper presents Stokesian dynamics simulations of experiments involving one or two charged colloids near either a single charged wall or confined between parallel charged walls. Equilibrium particle-particle and particle-wall interactions are interpreted from dynamic particle trajectories in simulations involving (1) a single particle levitated above a wall, (2) two particles below a wall, and (3) two particles confined between two parallel walls. By specifying only repulsive electrostatic Derjaguin-Landau-Verwey-Overbeek (DLVO) potentials and including multibody hydrodynamics, we successfully recover expected potentials in some cases, while anomalous attraction is observed in other cases. Attraction inferred in the latter simulations displays quantitative agreement with literature measurements when particle dynamics are interpreted using reported analyses. Because anomalous attraction is reproduced in simulations using only electrostatic repulsive DLVO potentials, our results reveal the one-dimensional analyses to be invalid for configurations that are inherently multidimensional via multibody hydrodynamics. Parameters related to experimental sampling of particle dynamics are also found to be critical for obtaining accurate potentials. We explain the anomalous attraction in each experiment using effective potentials, which can be employed in an a priori fashion to assist the confident design of future experiments involving interfacial and confined colloids. Ultimately, our findings reveal the importance of dimensionality and multibody hydrodynamics for understanding nonequilibrium dynamics of colloids near surfaces.     

On the solvation of the Zn2+ ion in methanol: a combined quantum mechanics, molecular dynamics, and EXAFS approach

The solvation properties of the Zn(2+) ion in methanol solution have been investigated using a combined approach based on molecular dynamics (MD) simulations and extended X-ray absorption fine structure (EXAFS) experimental results. The quantum mechanical potential energy surface for the interaction of the Zn(2+) ion with a methanol molecule has been calculated taking into account the effect of bulk solvent by the polarizable continuum model (PCM). The effective Zn-methanol interactions have been fitted by suitable analytical potentials, and have been utilized in the MD simulation to obtain the structural properties of the solution. The reliability of the whole procedure has been assessed by comparing the theoretical structural results with the EXAFS experimental data. The structural parameters of the first solvation shells issuing from the MD simulations provide an effective complement to the EXAFS experiments.     

Effective core potentials and the structures of metallocenes

The use of effective core potentials in the calculation of the geometrical parameters of the ferrocene molecule and its heavier analogs is reported. It is shown that a critical factor in these calculations is the efective core–core (ECC ) potential and, in the absence of ECC s for first-row atoms that are involved in short bonds, calculations of the geometrical parameters are not reliable. Good agreement with experimental geometries may be obtained by using the Los Alamos ECP s for atoms of the second and higher rows of the periodic table at the MP 2 level. DFT calculations have been performed and found to give numerical results comparable to MP 2 in the same basis. © 1995 John Wiley & Sons, Inc.     

新型燃速催化剂—烷基二茂铁桥联双聚分子的电化学研究

液态烷基二茂铁桥联双聚分子是一类新型燃速催化剂,它不仅具有催化活性高的优良特性,而且挥发性及迁移性均有改善,适宜用于火箭燃料复合固体推进剂。本文合成9种含不同取代基的双核二茂铁化合物,其中6种为首次合成。该9种化合物的通式为:其结构通式、各化合物的编号、英文名缩写及所含取代基和桥联基表示如下:     

Structures and vibrations of Nb3O and Nb3O-: a density functional study

Density functional calculations of neutral and anionic niobium trimer monoxides are presented. The calculations were performed employing scalar quasirelativistic effective core potentials. In order to test the accuracy of the used effective core potentials in the framework of density functional theory the pulsed field ionization-zero electron kinetic energy photoelectron spectrum of Nb(3)O was simulated and compared to experiment. Different isomers of Nb(3)O and Nb(3)O(-) were studied in order to determine the ground state structures. For both neutral and anionic systems a planar C(2v) structure with an edge-bound oxygen atom was found as a ground state. Equilibrium structure parameters, harmonic frequencies, and adiabatic electron affinity are reported. The calculated electron affinity and frequencies are in good agreement with the available experimental data obtained recently from vibrationally resolved negative ion photoelectron spectroscopy.     

Development and validation of an integrated computational approach for the study of ionic species in solution by means of effective two-body potentials. The case of Zn2+, Ni2+, and Co2+ in aqueous solutions

In this paper we have developed an effective computational procedure for the structural and dynamical investigation of ions in aqueous solutions. Quantum mechanical potential energy surfaces for the interaction of a transition metal ion with a water molecule have been calculated taking into account the effect of bulk solvent by the polarizable continuum model (PCM). The effective ion-water interactions have been fitted by suitable analytical potentials, and have been utilized in molecular dynamics (MD) simulations to obtain structural and dynamical properties of the ionic aqueous solutions. This procedure has been successfully applied to the Co2+-H2O open-shell system and, for the first time, Co-oxygen and Co-hydrogen pair potential functions have been determined and employed in MD simulations. The reliability of the whole procedure has been assessed by applying it also to the Zn2+ and Ni2+ aqueous solutions, and the structural and dynamical properties of the three systems have been calculated by means of MD simulations and have been found to be in very good agreement with experimental results. The structural parameters of the first solvation shells issuing from the MD simulations provide an effective complement to extended X-ray absorption fine structure (EXAFS) experiments.     

The solvation of Li and Na in acetonitrile from ab initio-derived many-body ion–solvent potentials

Several Li⁺- and Na⁺-acetonitrile models were derived from ab initio calculations at the counterpoise-corrected MP2/TZV++(d,p) level for distorted ion-(MeCN)_n clusters with n=1, 4 and 6. Two different many-body ion-acetonitrile models were constructed: an effective three-body potential for use with the six-site effective pair model of Böhm et al., and an effective polarizable many-body model. The polarizable acetonitrile model used in the latter model is a new empirical model which was also derived in the present paper. Mainly for comparative purposes, two ion-acetonitrile pair potentials were also constructed from the ab initio cluster calculations: one pure pair potential and one effective pair potential. Using all these potential models, MD simulations in the NPT ensemble were performed for the pure acetonitrile liquid and for Li⁺(MeCN) and Na⁺(MeCN) solutions with 1 ion in 512 solvent molecules and with a simulation time of at least 120 ps per system. Thermodynamic properties, solvation-shell structure and the self-diffusion coefficient of the ions and of the solvent molecules were calculated and compared between the different models and with experimental data, where available. The Li⁺ ion is found to be four-coordinated when the new many-body potentials are used, in contrast to the six-coordinated structure obtained for the pure pair and effective pair potentials. The coordination number of Na⁺ is close to six for all the models derived here, although the coordination number becomes slightly smaller with the many-body potentials. For both ions, the solvent molecules in the first shell point their nitrogen ends towards the cation, while in the second shell the opposite orientation is the most common.     

On the performance of DFT and interatomic potentials in predicting the energetics of (three-membered ring-containing) siliceous materials

We compare the enthalpies of transition for a range of SiO2 phases, including siliceous zeolites and dense polymorphs, calculated using three different interatomic potentials (Sanders-Leslie-Catlow (SLC), Sastre-Gale (SG), van Beest-Kramers-van Santen (BKS)), and from B3LYP periodic DFT calculations, with the experimentally measured values. It is found that the calculated results show a linear correlation with the measured values but that they often either considerably underestimate or overestimate enthalpy differences compared to experiment. Care should thus be taken when comparing experimental and calculated results. A linear rescaling of the calculated enthalpies to put the data on the same energy scale is proposed. Furthermore, it is found that when comparing enthalpies of transitions for materials containing three-membered rings, for which there is no experimental data available, the values, rescaled to the experimental energy scale, are very similar for both DFT and interatomic potentials (except for the BKS potential). The latter result suggests that the energetics of three-membered ring containing materials is well described using both approaches. Finally, we discuss the transition enthalpies of four three-membered ring containing siliceous materials and demonstrate that three-membered ring containing materials are not necessarily energetically disadvantageous but do become so progressively with increasing number of three-membered rings.     

Benchmarking of model core potentials: application to the halogen complexes of group 4 metals

The reliability of the model core potential (MCP) method was probed in a systematic RHF and MP2 study of the geometries of the group 4 metal halogen complexes (MX4; M = Ti, Zr, Hf and X = F, Cl, Br, I). The computed bond lengths were compared with experimental values, as well as those predicted using effective core potentials. Provided that electrons from the outermost core shell of the metal atom are treated explicitly in the calculation, both the MCP and ECP methods predict M-X bond lengths within 0.02-0.03 A of experiment. The reaction energies for a simple set of halogen substitution reactions of the MX4 complexes leading to the mixed halogen complex, MX2Y2, were also studied. Although no experimental values are available for these reactions, comparison was made with the values computed using effective core potentials. The predictability of the different pseudopotential techniques and the importance of the metal atom valence basis set contraction scheme and polarization space are discussed.     

Ab initio MRD-CI study of the spectrum of the TeO molecule employing relativistic effective core potentials

Potential energy curves and properties of the low-lying electronic states of tellurium oxide have been computed using a configuration interaction treatment that includes the spin-orbit coupling interaction. Relativistic effective core potentials (RECPs) are used to describe the inner shells of both the Te and O atoms. Good agreement is obtained for the spectroscopic constants of the X1-X2(3)sigma-, a1delta, and b1sigma+ states for which experimental data are available. The ratio of the parallel and perpendicular b-X transition moments, as well as the radiative lifetime of the b state, was computed, and both results were also found to be in good agreement with measurement. The energetic order of the electronic states in TeO appears to be very similar to that observed for the isovalent O2 molecule, but the Rydberg valence-mixing effects that are so prominent in the latter's spectrum (e.g., for the Schumann-Runge bands) are totally absent in TeO.     

Ab initio potential energy surface and spectrum of the B(3Pi) state of the HeI2 complex

The three-dimensional interaction potential for I2(B 3Pi0u+)+He is computed using accurate ab initio methods and a large basis set. Scalar relativistic effects are accounted for by large-core relativistic pseudopotentials for the iodine atoms. Using multireference configuration interaction calculations with subsequent treatment of spin-orbit coupling, it is shown for linear and perpendicular structures of the complex that the interaction potential for I2(B 3Pi0u+)+He is very well approximated by the average of the 3A' and 3A" interaction potentials obtained without spin-orbit coupling. The three-dimensional 3A' and 3A" interaction potentials are computed at the unrestricted open-shell coupled-cluster level of theory using large basis sets. Bound state calculations based on the averaged surface are carried out and binding energies, vibrationally averaged structures, and frequencies are determined. These results are found to be in excellent accord with recent experimental measurements from laser-induced fluorescence and action spectra of HeI2. Furthermore, in combination with a recent X-state potential, the spectral blueshift is obtained and compared with available experimental values.     

On the accuracy of density functional theory for iron-sulfur clusters

A simple, yet powerful wave function manipulation method was introduced utilizing a generalized ionic fragment approach that allows for systematic mapping of the wave function space for multispin systems with antiferromagnetic coupling. The use of this method was demonstrated for developing ground state electronic wave function for [2Fe-2S] and [Mo-3Fe-4S] clusters. Using well-defined ionic wave functions for ferrous and ferric irons, sulfide, and thiolate fragments, the accuracy of various density functionals and basis sets including effective core potentials were evaluated on a [4Fe-4S] cluster by comparing the calculated geometric and electronic structures with crystallographic data and experimental atomic spin densities from X-ray absorption spectroscopy, respectively. We found that the most reasonable agreement for both geometry and atomic spin densities is obtained by a hybrid functional with 5% HF exchange and 95% density functional exchange supplemented with Perdew's 1986 correlation functional. The basis set seems to saturate only at the triple-zeta level with polarization and diffuse functions. Reasonably preoptimized structures can be obtained by employing computationally less expensive effective core potentials, such as the Stuttgart-Dresden potential with a triple-zeta valence basis set. The extension of the described calibration methodology to other biologically important and more complex iron-sulfur clusters, such as hydrogenase H-cluster and nitrogenase FeMo-co will follow.     

Effective solvent mediated potentials of Na+ and Cl- ions in aqueous solution: temperature dependence

The effective solvent-mediated potentials for Na(+) and Cl(-) ions in aqueous solution were calculated in a wide range of temperatures from 0 to 100 °C. The potentials have been determined using the inverse Monte Carlo approach, from the ion-ion radial distribution functions computed in 50 ns molecular dynamics simulations of ions and explicit water molecules. We further separated the effective potentials into a short-range part and an electrostatic long-range part represented by a coulombic potential with some dielectric permittivity. We adjusted the value of the dielectric permittivity to provide the fastest possible decay of the short-range potentials at larger distances. The obtained temperature dependence of the dielectric permittivity follows well the experimental data. We show also that the largest part of the temperature dependence of the effective potentials can be attributed to the temperature-dependent dielectric permittivity.