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首页> 外文期刊>Chemical Physics: A Journal Devoted to Experimental and Theoretical Research Involving Problems of Both a Chemical and Physical Nature >Electrons and nuclei of ethylene isomers; a Feynman path integral - ab initio study
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Electrons and nuclei of ethylene isomers; a Feynman path integral - ab initio study

机译：乙烯异构体的电子和核;费曼路径积分-从头算

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摘要

The finite temperature properties of the ethylene isomers C_2H_4, C_2D_4, C_2~(13)H_4 and C_2~(13)D_4 have been studied by a Feynman path integral quantum Monte Carlo (PIMC) approach which has been combined with different electronic Hamiltonians. The nuclear potential V(R) in the PIMC step of the present formalism has been modeled by an efficient tight-binding one-electron Hamiltonian. Electronic expectation values in thermal equilibrium have been evaluated by ab initio Hartree-Fock and Moller-Plesset calculations. The quantum degrees of freedom of the ethylene nuclei as well as the anharmonicities in V(R) cause sizable elongations of the bond lengths relative to the hypothetical vibrationless values at the minimum of the potential energy surface. The PIMC results demonstrate impressively the wave-packet character of the nuclear wave function. This effect is neglected in the crude Born-Oppenheimer approximation which forms the basis of the large majority of electronic structure calculations of molecules. The nuclear degrees of freedom have a strong influence on the expectation values of the electronic Hamiltonian. The isotope and temperature dependence of these quantities has been analyzed. The nuclear fluctuations attenuate the nuclear-nuclear and electron-electron repulsions and lower the electronic energy. These stabilizing shifts in thermal equilibrium compete with a destabilization of the electron-nuclear attraction. The analysis of the ensemble averaged electronic quantities offers insight into the modifications of covalent bonding under the conditions of thermal equilibrium. Conceptual problems of classical Monte Carlo simulations as well as the shortcomings of electronic structure calculations on the basis of a single nuclear configuration in molecules with light atoms are emphasized. It is demonstrated that the nuclear degrees of freedom up to room temperature of the ethylene isomers studied are caused by quantum tunneling. Physical implications which follow from the present PIMC-ab initio investigation are mentioned concisely.
机译：乙烯异构体C_2H_4，C_2D_4，C_2〜（13）H_4和C_2〜（13）D_4的有限温度特性已通过费曼路径积分量子蒙特卡罗（PIMC）方法研究，并与不同的电子哈密顿量相结合。本形式主义的PIMC步骤中的核势V（R）已通过有效的紧束缚单电子哈密顿量建模。热平衡中的电子期望值已通过从头算起Hartree-Fock和Moller-Plesset计算进行了评估。乙烯核的量子自由度以及V（R）中的非谐性导致键长度相对于在势能面最小的假设无振动值有相当大的伸长。 PIMC结果令人印象深刻地展示了核波函数的波包特征。在粗大的Born-Oppenheimer近似中忽略了这种影响，该近似形成了大多数分子电子结构计算的基础。核自由度对电子哈密顿量的期望值有很大影响。分析了这些量的同位素和温度依赖性。核波动会减弱核-核和电子-电子的排斥力，并降低电子能。热平衡中的这些稳定变化与电子-核引力的不稳定相竞争。集合平均电子量的分析提供了对热平衡条件下共价键修饰的了解。强调了经典蒙特卡洛模拟的概念问题，以及在具有轻原子的分子中基于单个核构型的电子结构计算的缺点。结果表明，所研究的乙烯异构体在室温下的核自由度是由量子隧穿引起的。简要地提及了从目前的PIMC-ab从头进行的研究得出的物理含义。

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