Muñoz Macarena, Cárdenas Carlos
Departamento de Física, Facultad de Ciencias, Universidad de Chile, 653-Santiago, Chile.
Phys Chem Chem Phys. 2017 Jun 21;19(24):16003-16012. doi: 10.1039/c7cp02755a.
The chemical space contains all possible compounds that can be imagined. Its size easily equals the number of fundamental particles in the observable universe. Rational design of compounds aims to find those sectors of the chemical space where compounds optimize a set of desired properties. Then, rational design demands tools to efficiently navigate the chemical space. Ab initio alchemical derivatives offer the possibility to navigate, without empiricism, the energy landscape through alchemical transformations. An alchemical transformation is any process, physical or fictitious, that connects to points in the chemical space. In this work, those transformations are constructed as a perturbative expansion of the energy with respect to perturbations in the stoichiometry. The response functions of that expansion are what is called alchemical derivatives. In this work we assess how effective alchemical derivatives are in predicting energy changes associated to changes in the composition. We do this by including in the expansion, for the first time, electrostatic, polarization and electron-transfer effects. The system we chose is one that challenges alchemical derivatives because none of these effects dominates its behavior. The transmutations studied here correspond to substitutional doping of Al with up to four atoms of Si, AlSi. Two types of transformations are considered, those in which the number of electrons remains constant and those in which the number of electrons also changes. It is found that contrary to what has been reported before, polarization cannot be neglected. If polarization is not included, alchemical derivatives fail to predict the change of energy and the relative energy between isomers. For isoelectronic substitution of four or more atoms, the perturbative approach collapses because the strength of the perturbation becomes too strong to guarantee convergence of the series. It is shown, however, that if only one atom is mutated at a time, alchemical derivatives rank pretty well the isomers of AlSi according to their energy. In the case of non-isoelectronic transformations, it is observed that the series rapidly diverges with increasing number of electrons. In this situation, it becomes more important to keep the degree of transmutation of the parent system small.
化学空间包含了所有能够想象出来的化合物。其规模轻易就能等同于可观测宇宙中基本粒子的数量。化合物的理性设计旨在找出化学空间中那些化合物能优化一组所需性质的区域。因此，理性设计需要能有效在化学空间中导航的工具。从头算炼金术衍生物提供了一种无需经验主义就能通过炼金术变换在能量景观中导航的可能性。炼金术变换是指任何连接化学空间中两点的过程，无论是物理过程还是虚拟过程。在这项工作中，这些变换被构建为能量相对于化学计量学微扰的微扰展开。该展开的响应函数就是所谓的炼金术衍生物。在这项工作中，我们评估炼金术衍生物在预测与组成变化相关的能量变化方面的有效性。我们通过首次在展开中纳入静电、极化和电子转移效应来做到这一点。我们选择的系统对炼金术衍生物构成挑战，因为这些效应都没有主导其行为。这里研究的嬗变对应于用多达四个硅原子（AlSi）对铝进行替代掺杂。考虑了两种类型的变换，一种是电子数保持不变的变换，另一种是电子数也发生变化的变换。结果发现，与之前报道的情况相反，极化不能被忽略。如果不包括极化，炼金术衍生物就无法预测能量变化以及异构体之间的相对能量。对于四个或更多原子的等电子替代，微扰方法会失效，因为微扰强度变得太强以至于无法保证级数收敛。然而，结果表明，如果一次只突变一个原子，炼金术衍生物能根据能量很好地对AlSi的异构体进行排序。在非等电子变换的情况下，可以观察到随着电子数增加级数迅速发散。在这种情况下，保持母体系统的嬗变程度较小就变得更为重要。
