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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >The Theoreticla Prediction of Molecular Raadical Species: a Systematic Study of Equilibrium Geometries and Harmonic Vibrational Frequencies
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The Theoreticla Prediction of Molecular Raadical Species: a Systematic Study of Equilibrium Geometries and Harmonic Vibrational Frequencies

机译：分子自由基种类的理论预测：平衡几何和谐波振动频率的系统研究

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摘要

A systematic study of the accuracy of structures and frequencies of 33 small radical molecules is presented as predicted by Hartree-Fock (HF) theory, second-order Moller-Plesset (MP2) theory, coupled-cluster singles and doubles (CCSD) theory, coupled-cluster singles and doubles with perturbational triples correction [CCSD-(T)] theory, and gradient-corrected density functional theory with 3-parameter exact exchange mixing (B3LYP). For all methods, calculations were carried out using the Pople 6-31G~**, the correlation-consistent polarized valence double-#zeta# (cc-pVDZ), and the correlation-consistent polarized valence triple-#zeta# (cc-pVTZ) basis sets. While basis set effects were moderate, large differences in the performance of the different methods were found. Due primarily to artifactual symmetry breaking and orbital instabilities, both restricted and unrestricted HF and MP2 perform too erratically to be acceptable. CCSD with either restricted or unrestricted orbitals yields results in generally good agreement with experiment. However CCSD(T) geometries and frequencies exhibit a surprising lack of improvement and in many cases are less accrate than CCSD. The accuracy of B3LYP, however, is roughly comparable, or better, to CCSD and at much reduced computational cost and therefore is a good compromise between cost and accuracy for the routine study of molecular radicals. In addition, for several radicals significant discrepancies exist between the most reliable computational methods and existing experimental data for structures and frequencies.
机译：根据Hartree-Fock（HF）理论，二阶Moller-Plesset（MP2）理论，耦合集群单双打（CCSD）理论，对33种小自由基分子的结构和频率的准确性进行了系统的研究，耦合群单打和双打，采用微扰三重校正[CCSD-（T）]理论，以及梯度校正密度泛函理论和3参数精确交换混合（B3LYP）。对于所有方法，都使用Pople 6-31G〜**，相关一致的极化价double-＃zeta＃（cc-pVDZ）和相关一致的极化价Triple-＃zeta＃（cc- pVTZ）基础集。虽然基集效果中等，但发现不同方法的性能差异很大。主要由于人为的对称性破坏和轨道不稳定性，受限制的和不受限制的HF和MP2的表现都过于不稳定，无法接受。具有受限制轨道或不受限制轨道的CCSD产生的结果通常与实验吻合良好。但是，CCSD（T）的几何形状和频率显示出令人惊讶的改进不足，并且在许多情况下不如CCSD准确。但是，B3LYP的准确性与CCSD大致相当或更好，并且计算成本大大降低，因此是常规研究分子自由基的成本与准确性之间的良好折衷。此外，对于几个基本原理，最可靠的计算方法与结构和频率的现有实验数据之间存在重大差异。

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