First principles calculation for hydrogen/positronium adsorption on an Si(111) surface using the dynamical extended molecular orbital method

We have recently proposed the dynamical extended molecular orbital (DEMO) method, which can be applied to the adsorption of light particles, such as positrons, muons, protons, deuterons, and tritrons, on an Si(111) surface, in order to explore theoretically the isotope effect on their structures, wave functions, and energies. Since the DEMO method determines both electronic and light particles' wave functions simultaneously by optimizing all parameters including basis sets and their centers variationally, we have obtained different orbitals for light particles as well as their electronic wave functions. In the case of positron adsorption, we have found that the chemisorption of positronium (Ps) occurs on the Si(111) surface, being stabilized by the relaxation of electronic wave functions near the dangling bond. The calculated Ps adsorption energy is in good agreement with the experimental value. The isotope effect of light particles is also clearly observed on the Si(111) surface. The present results are promising in the sense that the DEMO method is a very useful tool to demonstrate the dynamics of light particles on a surface. (C) 1999 Published by Elsevier Science B.V. All rights reserved.