Reactions on surfaces are often modeled using molecular clusters which are too small to accurately represent the mechanical environment of bulk materials. The small size of these clusters is driven by the large cost of ab initio quantum mechanical (QM) computational methods needed to accurately model chemical reactions. Hybrid computational approaches that interface quantum mechanics with molecular mechanics (MM) methods, commonly referred to as QM/MM methods, are becoming increasingly popular for treating large systems, but these hybrid methods have not been applied to surface models. This paper presents a QM/MM optimization scheme for modeling surfaces that is based on the IMOMM approach of Maseras and Morokuma. The modified method, (S)urface IMOMM, and its applications to surface chemistry are discussed. Disciplines Chemistry CommentsReprinted (adapted) ReceiVed: June 12, 1998; In Final Form: NoVember 20, 1998 Reactions on surfaces are often modeled using molecular clusters which are too small to accurately represent the mechanical environment of bulk materials. The small size of these clusters is driven by the large cost of ab initio quantum mechanical (QM) computational methods needed to accurately model chemical reactions. Hybrid computational approaches that interface quantum mechanics with molecular mechanics (MM) methods, commonly referred to as QM/MM methods, are becoming increasingly popular for treating large systems, but these hybrid methods have not been applied to surface models. This paper presents a QM/MM optimization scheme for modeling surfaces that is based on the IMOMM approach of Maseras and Morokuma. The modified method, (S)urface IMOMM, and its applications to surface chemistry are discussed.

The MNDO Hamiltonian as incorporated within MOPAC has been utilized to predict dynamics for some simple reactions. In one option, the intrinsic reaction coordinate has been followed along the path of steepest descent from the transition state backward to reactants and forward to products. In a second option, dynamics of isolated molecular systems have been calculated. In each case, the potential surface (as predicted by the MNDO Hamiltonian) is calculated in situ as the atomic trajectories are calculated from Newton's Laws of Motion. Several specific examples are given and discussed.

We have demonstrated that a wide variety of pentacoordinate silicon anions (siliconates) should be stable and can be prepared by combining the predictive powers of MNDO and ab initio computational methods and the flowing afterglow (FA) experimental technique. MNDO has been used to compute the anion affinities of 91 siliconates; all but five of these are predicted to be stable with respect to the loss of an anion. Twenty-four siliconates, most of them previously unreported, have been prepared and studied in the FA. The MNDO predictions were, in general, consistent with the experimental results and with trends previously reported by Corriu and co-workers, but in some cases they were found deficient. For example,

Ab initio calculations, employing double zeta plus polarization (DZP) basis sets and generalized valence bond (GVB) wave functions, have been performed on clusters of varying size, to investigate the utility of such clusters as prototypes for the study of siliconsurfaces, and to investigate the effect of the level of theory used on predicted results. This work builds on landmark papers by Goddard in 1982 and Paulus in 1998 that demonstrate that a single reference wave function description of the silicon dimer bond is incorrect, and that a multireference description results in a symmetric dimer in a silicon cluster containing one dimer. In this work, it is shown that the imposition of arbitrary geometrical constraints (fixing subsurface atoms at lattice positions) on cluster models of the Si(100) surface can also lead to nonphysical results. Calculations on the largest clusters, without geometrical constraints, reveal that surface rearrangement due to dimer bond formation is "felt" several layers into the bulk. The predicted subsurface displacements compare favorably to experiment. Thus, small clusters, such as Si9H12, cannot adequately represent bulk behavior. Vibrational analysis shows that dimer buckling modes require minimal excitation energy, so the experimental observation of buckled dimers on siliconsurfaces may reflect the ease with which a symmetric dimer can be perturbed from its minimum energy structure. In the study of surface reconstruction and relaxation, and the associated issue of the buckling of dimer surfaces, it is critical to use adequate wave functions. As shown in this work and previously by Goddard and Paulus, this generally means that multireference treatments are needed to correctly treat the dangling bonds.

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