Oligovalent link atoms in embedding calculations.

Making reasonable statements about large systems is a notoriously difficult task of quantum chemistry. If such a system consists of a small but electronically important region, the core, which requires a treatment on a high level of theory, and a larger remainder, the bulk, which only acts in a perturbative fashion and thus admits a description on a lower level of theory, then it is appropriate to partition it accordingly and use an embedding scheme to provide for the coupling of the two regions. In many cases of said partitioning it will be necessary to cut covalent bonds so that the emerging free valences have to be saturated by link atoms. Mostly hydrogen atoms are used. However, it may be necessary to separate the entire system so that bulk atoms are generated that are bound to two or three core atoms at once, that is, such bulk atoms are divalent or even trivalent with respect to the core. In the present article we demonstrate by means of several silane molecules, ranging from Si3H8 to Si104H92, that the use of oligovalent link atoms is quite promising.