Geometries and energy separations of various low-lying electronic states of an iron trimer (Fe₃) are investigated by coupled-cluster singles and doubles (CCSD) and coupled-cluster singles and doubles plus perturbative triples [CCSD(T)] calculations. The ground state is found to be a ¹³A^′ state with C_s symmetry, whereas a nearly isoenergetic state, ¹³A₁ (C_2v), is degenerate to the ground state. The ground and five low-lying states with a spin multiplicity of 13 are found below 0.20 eV at the CCSD(T) level. On the other hand, the low-lying states with spin multiplicities of 9, 11, and 15 appear only above 0.20 eV. From detailed natural bond orbital analyses, Fe₃ has Fe–Fe bonds composed of $\mathrm{\sigma }$-bond orbitals only in the$\mathrm{\beta }$-spin part with higher s-character in low-lying states with a spin multiplicity of 13. The polarization coefficients indicate that the $\mathrm{\sigma }$_Fe–Fe bonds are nearly complete covalent bonds with little polarization.

中文翻译：

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通过CCSD和CCSD（T）计算铁三聚体（Fe3）的几何形状和电子状态

铁三聚体（Fe ₃）的各种低电子态的几何构型和能量分离通过耦合簇单双打（CCSD）和耦合簇单双打加上扰动三重[CCSD（T）]计算进行研究。发现基态是具有C _s对称性的¹³ A ^′状态，而几乎是等能量态¹³ A ₁（C _2v），退化为基态。在CCSD（T）电平下，发现自旋多重性为13的基态和五个低洼状态低于0.20 eV。另一方面，自旋多重性为9、11和15的低洼状态仅出现在0.20 eV以上。根据详细的自然键轨道分析，Fe _3的Fe–Fe键由$\mathrm{\sigma }$-仅在$\mathrm{\beta }$低自旋状态下具有较高s特性的自旋部分，自旋多重性为13。极化系数表明， $\mathrm{\sigma }$_Fe–Fe键几乎是完全共价键，几乎没有极化。