Semiempirical quantum mechanical (SEQM) methods offer an attractive middle ground between fully ab initio quantum chemistry and force-field simulations, allowing for a quantum mechanical treatment of the system at a relatively low computational cost. However, SEQM methods have not been frequently utilized in the study of transition metal systems, mostly due to the difficulty in obtaining reliable parameters. This paper examines the accuracy of the PM6 and PM7 semiempirical methods to predict geometries, ionization potentials, and HOMO–LUMO energy gaps of several bare gold clusters (Au_n) and thiolate-protected gold nanoclusters (AuSNCs). Contrary to PM6, the PM7 method can predict qualitatively correct geometries and ionization potentials when compared to DFT. PM6 fails to predict the characteristic gold core and gold–sulfur ligand shell (staple motifs) of the AuSNC structures. Both the PM6 and PM7 methods overestimate the HOMO–LUMO gaps. Overall, PM7 provides a more accurate description of bare gold and gold–thiolate nanoclusters than PM6. Nevertheless, refining the gold parameters could help achieve better quantitative accuracy.

中文翻译：

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裸露和硫醇保护的金纳米团簇上PM6和PM7方法的准确性

半经验量子力学（SEQM）方法在完全从头开始的量子化学和力场模拟之间提供了有吸引力的中间立场，从而允许以相对较低的计算成本对系统进行量子力学处理。但是，SEQM方法在过渡金属系统的研究中并未得到广泛应用，这主要是由于难以获得可靠的参数。本文研究了PM6和PM7半经验方法预测几个裸金团簇的几何形状，电离势和HOMO-LUMO能隙的准确性（Au _n）和硫醇盐保护的金纳米团簇（AuSNC）。与PM6相反，与DFT相比，PM7方法可以定性地预测正确的几何形状和电离势。PM6无法预测AuSNC结构的特征性金核和金硫配体壳（主图案）。PM6和PM7方法都高估了HOMO-LUMO的差距。总体而言，与PM6相比，PM7对裸金和硫醇金纳米簇的描述更为准确。然而，提炼金参数可以帮助实现更好的定量准确性。