Heidar-Zadeh Farnaz, Ayers Paul W, Bultinck Patrick
Department of Chemistry, Ghent University, Krijgslaan 281 (S3), 9000, Ghent, Belgium.
Center for Molecular Modeling, Ghent University, Technologiepark 903, 9052, Zwijnaarde, Belgium.
J Mol Model. 2017 Nov 21;23(12):348. doi: 10.1007/s00894-017-3514-6.
Atoms in molecules methods that rely on reference promolecular densities typically require that one define, or otherwise determine, the densities of unbound atomic anions. Whereas the isolated atomic polyanions are always physically and computationally unbound, monoanions can be either physically bound but computationally unbound (like the oxygen anion at the Hartree-Fock level of theory), or physically unbound but computationally bound (like the nitrogen anion using many DFT methods with a basis set including diffuse functions). Depending on the level of theory and basis set used, the densities of negatively charged atomic ions can decay very slowly and even be nonmonotonically decreasing. These delocalized anionic densities induce ill-behaved atomic properties for compounds containing highly reduced atoms. To treat the problem of unphysical proatom densities in iterative Hirshfeld methods, we compute the smallest (typically fractional) nuclear charge to bind all electrons, called the effective nuclear charge [Formula: see text] of an atom A. When [Formula: see text] at a given level of theory, the scaled density corresponding to the effective nuclear charge is used as the negatively charged proatom density. This novel approach dramatically improves the computational robustness of the iterative Hirshfeld partitioning scheme.
分子中的原子方法通常依赖于参考前分子密度，这通常要求定义或以其他方式确定未结合原子阴离子的密度。虽然孤立的原子多阴离子在物理和计算上总是未结合的，但单阴离子可以是物理上结合但计算上未结合的（如在Hartree-Fock理论水平下的氧阴离子），或者是物理上未结合但计算上结合的（如使用许多带有包含弥散函数基组的DFT方法的氮阴离子）。根据所使用的理论水平和基组，带负电原子离子的密度可能衰减非常缓慢，甚至呈非单调递减。这些离域的阴离子密度会导致含高度还原原子的化合物出现不良的原子性质。为了处理迭代Hirshfeld方法中不合理的原原子密度问题，我们计算结合所有电子所需的最小（通常是分数）核电荷，即原子A的有效核电荷[公式：见正文]。当在给定理论水平下[公式：见正文]时，对应于有效核电荷的缩放密度用作带负电的原原子密度。这种新方法显著提高了迭代Hirshfeld划分方案的计算稳健性。
