闭壳极小值间开壳过渡态势垒高度的评定困难:以小C4n环为例。

G. David, N. Ben Amor, Tao Zeng, N. Suaud, G. Trinquier, J. Malrieu
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引用次数: 2

摘要

C4n环芳烃在平衡几何上表现出强烈的键交替。在两个等效几何中,系统基本上保持闭壳特性。两个能量最小值被一个抑制键交替的过渡态分开,其中波函数是强烈的二自由基。本文讨论了与能量差有关的物理因素以及对势垒高度的可能评价。平衡态的受限密度泛函理论(DFT)/B3LYP与过渡态的破对称DFT/B3LYP之间的能量差给出的势垒要么是负的,要么是小的,这与最可靠的波函数理论计算相矛盾。最小(两个分子轨道上的两个电子)完全主动空间自洽场(CASSCF)高估了势垒,随后的二阶扰动抵消了它。由于自旋极化效应的集体性,需要进行全π CASSCF +二阶微扰才能达到合理的势垒值,但这种处理方式不适用于大分子。在相同的脚上处理闭壳和开壳几何的DFT方法已经被探索,如混合参考自旋-翻转时间依赖-DFT和一个新的自旋去污建议,即DFT修饰的组态相互作用,但结果仍然依赖于密度泛函。不带或带自旋去污的M06-2X与精确的波函数结果最吻合。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Difficulty of the evaluation of the barrier height of an open-shell transition state between closed shell minima: The case of small C4n rings.
C4n cyclacenes exhibit strong bond-alternation in their equilibrium geometry. In the two equivalent geometries, the system keeps an essentially closed-shell character. The two energy minima are separated by a transition state suppressing the bond-alternation, where the wave function is strongly diradical. This paper discusses the physical factors involved in this energy difference and possible evaluations of the barrier height. The barrier given as the energy difference between the restricted density functional theory (DFT)/B3LYP for the equilibrium and the broken symmetry DFT/B3LYP of the transition state is either negative or small, in contradiction with the most reliable Wave Function Theory calculations. The minimal (two electrons in two molecular orbitals) Complete Active Space self-consistent field (CASSCF) overestimates the barrier, and the subsequent second-order perturbation cancels it. Due to the collective character of the spin-polarization effect, it is necessary to perform a full π CASSCF + second-order perturbation to reach a reasonable value of the barrier, but this type of treatment cannot be applied to large molecules. DFT procedures treating on an equal foot the closed-shell and open-shell geometries have been explored, such as Mixed-Reference Spin-Flip Time-dependent-DFT and a new spin-decontamination proposal, namely, DFT-dressed configuration interaction, but the results still depend on the density functional. M06-2X without or with spin-decontamination gives the best agreement with the accurate wave function results.
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