| Our recent QMC work on self-interstitial defects in silicon [PDF] and on small carbon clusters [PDF] showed that conventional methods such as Kohn-Sham density functional theory within the local density and generalised gradient approximations are often surprisingly inaccurate when comparing very different types of chemical bonding. This observation has serious implications for much theoretical work on surfaces. We are currently using QMC to calculate the surface energies of jellium (about which there is a current controversy [1] and Al(111). This will allow us to make an initial estimate of the accuracy of conventional techniques used to study surfaces, but the more ambitious project envisaged here will tell us much more. It is now well established experimentally [2] that at reasonably high coverages Na adsorbs substitutionally on both the Al(001) and Al(111) surfaces. In other words, the Na atoms move into the top layer of the surface, taking the place of Al atoms. The energy balance leading to substitutional absorption is subtle, involving the comparison of Na atoms and Al atoms bonded in very different ways, but appears to be reasonably well accounted for by local DFT. We are somewhat sceptical of the extent to which this agreement is quantitative, however, and plan to investigate using QMC calculations of the Al(001) surface. In particular, we will compare the energies of the 2x2 substitutional and on-surface hollow-site adsorption structures. |
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[2] See the reviews in Surf. Rev. Lett. 2, 315 (1995). |