We have investigated adsorption of a single H atom onto a charge-neutral B&2H» cluster, which simulates one of the subunits of an icosahedral boride. Five different paths of approach are considered for the H atom. At the level of fourth-order Moiler-Plesset perturbation theory, only one of these paths leads to an equilibrium position in which the adsorption energy is positive. Although H can pass inside the icosahedral B» cage, the most stable position is outside the cage, corresponding approximately to the position for which electronic charge transfer to the 8» cage is a maximum. A comparison is carried out between the charge densities and one-electron energy levels of the cluster in the absence and presence of the adsorbate.

New and more accurate energy profiles for interstitial muonium in diamond clusters (C10H16 to C30H40) have been determined by ab initio and approximate ab initio calculations, and have been critically compared to previous theoretical work. The energy profiles are shown to be extremely sensitive to the position of the surface atoms used to terminate the cluster. It is established that if the equilibrium carbon-hydrogen bond length is used to terminate the cluster the energy profiles converge rapidly with cluster size, but the use of any other value for d C -n leads to qualitatively incorrect energy profiles for small clusters.

Estreicher et al Respond:The calculation of properties of defects in semiconductors with use of clusters and the ab initio unrestricted Hartree-Fock (UHF) method requires the calculation of the order of A'^ integrals (iV is the number of orbitals) and thus is limited to small clusters. It is essential to choose and terminate the cluster in such a way that the calculated properties of the defect are independent of its surface. If hydrogen atoms are used to saturate the dangling bonds one cannot choose the C-H bond length (^CH) arbitrarily: Its value must be such that results independent of the size of the cluster are obtained. Sahoo, Mishra, and Das (SMD) propose to allow the C-H distance to vary in order to reproduce the experimental average band gap. This procedure introduces a semiempirical parameter (rfce) and is cluster-size dependent.As noted by SMD, the CioHieMu cluster with den "=1.09 A leads to a contact density at the muon larger than the one observed, and a calculated band gap larger than the one obtained with rfcH'"L54 A, and predicts a small excess charge on some host atoms. We will respond to these and other arguments as numbered by SMD.1. In our previous Letter,^ we demonstrated that energy profiles independent of cluster size are obtained if and only if dcH^\.Q9 A. We have now completed^ calculations of spin densities for a number of clusters using the partial retention of diatomic differential overlap (PRDDO) method and ab initio UHF method and have found the following results for the enhancement factor / (spin density at the ^ "*" divided by its value in vacuum). Without averaging over the zero-point motion of the muon and with the muon at the tetrahedral interstitial (r) site, we get with PRDDO and rfcH"l-54 A, for CioHie, /-1.03; for C20H32, /«1.20; for C30H40, /«1.32; with rfcH-1.09 A, for C10H16, /-1.45; for C20H32, /-1.40; for C30H40, /-1.38. Ab initio UHF with the CtoHi6 cluster gives /-0.91 if rfcH-1.54 A and / =* 1.25 if dew "" 1 09 A. The contact densities obtained with PRDDO are slightly larger than the ab initio values, probably because PRDDO uses Slater orbitals rather than Gaussians. The dependence of / on cluster size is much larger if den " 1.54 A than if den "1.09 A. Both values converge to an / close to the one obtained with C10H16 and rfcH*"109 A. The physical origin of the reduced contact density for C10H16 with rfcH'"L54 A is that the C-H bonds are insufficiently saturated and the electronic wave function on the muon itself contributes to saturate these bonds. This mechanism does lower the contact density but is not representative of the experimental situation and is cluster-size dependent. Further, the contact density becomes extremely sensitive to the zero-point motion of the muon, the importance of which is overestimated by Sahoo et al^ The experimental value of the contact density at the muon cannot be reproduced at the Hartree-Fock (HF) level with the CioHi6Mu cluster if t/cH'"1.09 A (even with very large