Quantum saturating dangling bonds

Another possibility is to saturate dangling bonds of the representative cluster with hydrogen atoms. The prototype molecule thus obtained is relatively easy to handle with standard quantum chemical methods, and one can hope that the artefact effects are small or negligible, if the model system is sufficiently large. In a few cases, comparison of the electronic charge density maps in the finite models saturated by hydrogen atoms and the similar maps in accurate three-dimensional models confirm the use of prototype molecules [64]. 

First attempts to saturate dangling bonds by effective potentials (quantum capping potentials) were performed by DiLabio et al. [390, 391, 396] based on analytical effective core potentials of the Goedecker type [397, 398]. In line with... 

The simplest way to keep the electronic structure of the quantum subsystem as close as possible to what it would be in the entire macromolecule consists of saturating the dangling bonds with monovalent atoms called link atoms. Typically, hydrogen atoms are used. The computation now consists of a model molecule of the reactive part interacting with classical surroundings, similar to the case of solutions. This approach has been introduced by Singh and Kollman [8] and has been put in a operational form by Field et al. [9],... 

In the prototype molecule approach the dangling bonds are usually saturated by hydrogen atoms. This procedure, although not without controversy, offers a simple recipe for obtaining closed shell model molecules, with well-defined electronic states, easily tractable with quantum chemical methods. 

The cage system is treated quantum mechanically. In the original version of the model all valence electrons were included and to allow a natural definition of the cage, orthogonalized atomic hybrid orbitals were used as a basis set [215]. This allows to avoid problems with the saturation of dangling bonds since all hybrids on the same atom may belong to the cage with a wave function obtained by solution of a closed-shell secular equation. 

The cluster approach is an alternative way to make quantum mechanical calculations on solids feasible. The microscopically infinite solid is replaced by a finite piece and the dangling bonds are saturated by hydrogen atoms. The advantage is that molecular quantum chemical ab initio codes can be immediately applied. Of course, some care is necessary... 

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