Spin–spin coupling constants and triplet instabilities in Kohn–Sham theory

Spin–spin coupling constants and triplet instabilities in Kohn–Sham theory
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Kohn-Sham 理论中的自旋-自旋耦合常数和三重态不稳定性

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发表时间：
2010
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通讯作者：
M. Jaszuński
M. Jaszuński
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作者：
Ola B. Lutnæs;T. Helgaker;M. Jaszuński

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使用限制性哈特里-福克理论计算的间接核自旋-自旋耦合常数是不可靠的，因为通常占主导地位的费米接触（FC）贡献来自电子系统的三重态扰动，在哈特里-福克理论中描述得很差-特别是在接近三重态不稳定性开始的几何结构中。这些问题通常但并非总是在Kohn-Sham理论中克服，该理论通常提供良好的自旋-自旋耦合常数。在这里，我们检查自旋-自旋耦合常数的灵敏度，三重态不稳定性在Kohn-Sham和Hartree-Fock理论相关的质量的自旋-自旋耦合常数和质量的最低三重态激发能的一些小分子。一般来说，FC贡献在局域密度近似（LDA）中最稳定，在广义梯度近似（GGA）中稍不稳定;另一方面，GGA耦合常数通常比LDA常数更准确。重要的是，虽然杂化理论通常比GGA理论给出更好的结果，但它也更容易受到三重态不稳定性的影响（继承了哈特里-福克理论的一些问题），因此在自旋-自旋耦合常数方面不如GGA理论可靠。对于自旋-自旋耦合常数的计算，我们推荐Perdew-Burke-Ernzerhof GGA交换相关泛函，它提供了准确性和鲁棒性的良好折衷。
Indirect nuclear spin–spin coupling constants calculated using restricted Hartree–Fock theory are unreliable since the usually dominant Fermi-contact (FC) contribution arises from a triplet perturbation of the electronic system, poorly described in the Hartree–Fock theory – in particular, at geometries close to the onset of triplet instabilities. These problems are usually but not invariably overcome in Kohn–Sham theory, which typically provides good spin–spin coupling constants. We here examine the sensitivity of spin–spin coupling constants to triplet instabilities in Kohn–Sham and Hartree–Fock theories by correlating the quality of the spin–spin coupling constants and the quality of the lowest triplet excitation energy for a number of small molecules. In general, the FC contributions are most stable in the local density approximation (LDA) and slightly less stable in the generalised gradient approximation (GGA); on the other hand, GGA coupling constants are usually more accurate than LDA constants. Importantly, although hybrid theory often gives better results than the GGA theory, it is also more susceptible to triplet instabilities (inheriting some of the problems of the Hartree–Fock theory) and therefore less reliable than the GGA theory for spin–spin coupling constants. For calculations of spin–spin coupling constants, we recommend the Perdew–Burke–Ernzerhof GGA exchange-correlation functional, which provides a good compromise of accuracy and robustness.