Splitting multiple bonds: A comparison of methodologies on the accuracy of bond dissociation energies

A benchmarking of different quantum chemical methodologies for the splitting of multiply bonded systems is presented, with an emphasis on quantitative reproduction of experimentally determined dissociation energies. New benchmark full configuration interaction (FCI) calculations are presented for nitrogen and acetylene, and comparisons are made between various methods with both the FCI results and with experiment in an effort to understand qualitatively and quantitatively how well these different methods cope with the bond‐breaking process. It is found that the multireference methods fare well, but are sometimes slowly convergent with respect to the size of the basis set, and in particular the angular momenta of the polarization functions. Single‐reference (SR) wavefunction‐based methods perform poorly against full CI, while CCSD(T) is reasonably accurate for comparison with experiment. Density functional theory with hybrid functionals have very quick basis‐set convergence, and offer reliable estimates of bond dissociation energies. Spin‐restricted SR wavefunctions are found to be poor for such systems, while unrestricted wavefunctions give reasonably good energies, but with severe spin contamination. © 2013 Wiley Periodicals, Inc.     

Full configuration interaction calculation of the low lying valence and Rydberg states of BeH

The all-electron full configuration interaction (FCI) vertical excitation energies for some low lying valence and Rydberg excited states of BeH are presented in this article. A basis set of valence atomic natural orbitals has been augmented with a series of Rydberg orbitals that have been generated as centered onto the Be atom. The resulting basis set can be described as 4s2p1d/2s1p (Be/H) + 4s4p3d. It allows to calculate Rydberg states up to n= {3,4,5} of the s, p, and d series of Rydberg states. The FCI vertical ionization potential for the same basis set and geometry amounts to 8.298 eV. Other properties such as FCI electric dipole and quadrupole moments and FCI transition dipole and quadrupole moments have also been calculated. The results provide a set of benchmark values for energies, wave functions, properties, and transition properties for the five electron BeH molecule. Most of the states have large multiconfigurational character in spite of their essentially single excited nature and a number of them present an important Rydberg-valence mixing that is achieved through the mixed nature of the particle MO of the single excitations.     

The NMR indirect nuclear spin–spin coupling constant of the HD molecule

We present new calculated and experimental values of the NMR indirect nuclear spin–spin coupling constant in HD. In the quantum-chemical ab initio calculations, the full configuration-interaction (FCI) method is used, yielding an equilibrium value of 41.22?Hz in the basis-set limit. Adding a calculated zero-point vibrational correction of 1.89?Hz and a temperature correction of 0.20?Hz at 300?K, we obtain a total calculated spin–spin coupling constant of J _FCI(HD)?=?43.31(5)?Hz at 300?K. This result is within the error bars of the experimental gas-phase NMR value, J _exp(HD)?=?43.26(6)?Hz, obtained by extrapolating values measured in HD–He mixtures to zero density.     

Direct simulation of MHD flows in dual-coolant liquid metal fusion blanket using a consistent and conservative scheme

Direct simulation of 3-D MHD (magnetohydrodynamics) flows in liquid metal fusion blanket with flow channel insert (FCI) has been conducted. Two kinds of pressure equilibrium slot (PES) in FCI, which are used to balance the pressure difference between the inside and outside of FCI, are considered with a slot in Hartmann wall or a slot in side wall, respectively. The velocity and pressure distribution of FCI made of SiC/SiC_f are numerically studied to illustrate the 3-D MHD flow effects, which clearly show that the flows in fusion blanket with FCI are typical three-dimensional issues and the assumption of 2-D fully developed flows is not the real physical problem of the MHD flows in dual-coolant liquid metal fusion blanket. The optimum opening location of PES has been analyzed based on the 3-D pressure and velocity distributions.     

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实验研究了通道插件(FCI)压力平衡孔(PEH)和压力平衡狭缝(PES)结构、绝缘和非绝缘材质对磁流体动力学(MHD)效应的影响,结果表明：带PEH或PES、绝缘或非绝缘材质的FCI产生的流速分布有很大的差异;对MHD压降而言,FCI结构PEHs或PES差异的影响大于FCI绝缘或非绝缘材质差异引起的影响;对流速分布而则刚好相反。二次流引起的局部流速骤增的MHD效应(简称为S-MHD效应)对控制管道内流速分布和降低MHD压降有作用,这些实验结果为弄清FCI的MHD效应机制和液态金属包层设计提供了实验认知与有价值的参考。     

Iterative submatrix diagonalisation for large configuration interaction problems

The Davidson method has been highly successful for solving for eigenpairs of the large matrices that are common in quantum chemical simulations. Electronic structure simulations, however, can still easily generate matrices that are too large for current computational resources to handle. Therefore, many strategies have arisen to obtain eigenpairs of sufficient accuracy without considering the full Hamiltonian matrix. This article introduces one such strategy by creating a systematic series of submatrix approximations to the full matrix using natural orbitals. By solving for eigenpairs in this series, the eigenvalue accuracy can be gradually increased until a convergence threshold is reached. Importantly, this allows the series to terminate without ever reaching the full matrix, resulting in lower computational costs and reduced memory demands. Application of the method to the full configuration interaction problem for ground states, excited states and potential energy scans of various systems shows that the iterative submatrix diagonalisation method can systematically control eigenvalue errors and provide substantial cost-savings. This method is therefore expected to be highly useful for large-scale diagonalisation problems in electronic structure theory.     

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给出了通道插件管道MHD效应的初步实验结果,结果表明：中心区流速分布与数值预测的差别较大,且不同位置的管道截面流速分布不同,在压力平衡孔之间呈周期性变化;宏观的中心区与边缘区流量分配、MHD压降与简化理论预测结果相接近。这些数据将给FCI管道数值分析模型的完善和液态包层设计提供参考。     

燃料与冷却剂相互作用的数值模拟

本文通过对MIXA06实验的模拟来研究发生在核反应堆严重事故情况下的燃料与冷却剂相互作用(FCIs)细粒化混合过程.采用基于泰勒不稳定性的水力学细粒化模型,对这种高温熔融物与低温冷却剂接触时的强烈热物理作用过程进行数值计算和预测,研究在低韦伯数条件下水力学细粒化模型的适用性.计算结果和参数敏感性分析结果表明,水力学模型低估了熔融液滴的细粒化率.