Midbond basis functions for weakly bound complexes

Weakly bound systems present a difficult problem for conventional atom-centred basis sets due to large separations, necessitating the use of large, computationally expensive bases. This can be remedied by placing a small number of functions in the region between molecules in the complex. We present compact sets of optimised midbond functions for a range of complexes involving noble gases, alkali metals and small molecules for use in high accuracy coupled -cluster calculations, along with a more robust procedure for their optimisation. It is shown that excellent results are possible with double-zeta quality orbital basis sets when a few midbond functions are added, improving both the interaction energy and the equilibrium bond lengths of a series of noble gas dimers by 47% and 8%, respectively. When used in conjunction with explicitly correlated methods, near complete basis set limit accuracy is readily achievable at a fraction of the cost that using a large basis would entail. General purpose auxiliary sets are developed to allow explicitly correlated midbond function studies to be carried out, making it feasible to perform very high accuracy calculations on weakly bound complexes.弱束缚体系对于传统以原子为中心的基组（atom-centred basis sets）而言是一类极具挑战性的研究对象，由于体系内分子间距较大，必须采用规模庞大、计算成本高昂的基组进行计算。通过在复合物内的分子间区域添加少量基函数，可有效解决这一难题。本研究针对一系列涉及稀有气体、碱金属与小分子的复合物，构建了经过优化的紧凑键间函数（midbond functions）集合，用于高精度耦合簇（coupled-cluster）计算，并提出了一套更为稳健的基函数优化流程。研究表明，在双ζ质量轨道基组中添加少量键间函数后，即可获得优异的计算结果，可将一系列稀有气体二聚体的相互作用能与平衡键长的计算精度分别提升47%与8%。当与显式关联方法（explicitly correlated methods）结合使用时，仅需付出使用大型基组所需成本的极小一部分，即可轻松实现近乎完备基组极限（complete basis set limit）的计算精度。本研究还开发了通用辅助基组，以支持键间函数的显式关联研究，使得对弱束缚复合物开展超高精度计算成为可能。