We report a first-principles theoretical study of hyperfine interactions, zero-point effects, and defect energetics of muonium and hydrogen impurities in silicon and germanium. The spin-polarized density-functional method is used, with the crystalline orbitals expanded in all-electron Gaussian basis sets. The behavior of hydrogen and muonium impurities at both the tetrahedral and bond-centered sites is investigated within a supercell approximation. To describe the zero-point motion of the impurities, a double adiabatic approximation is employed in which the electron, muon/proton, and host lattice degrees of freedom are decoupled. Within this approximation the relaxation of the atoms of the host lattice may differ for the muon and proton, although in practice the difference is found to be slight. With the inclusion of zero-point motion the tetrahedral site is energetically preferred over the bond-centered site in both silicon and germanium. The hyperfine and superhyperfine parameters, calculated as averages over the motion of the muon, agree reasonably well with the available data from muon spin resonance experiments. ͓S0163-1829͑99͒01643-4͔

The excitonic states of four small hydrogenated Si clusters (SiH 4 ,Si 2 H 6 ,Si 5 H 12 , and Si 10 H 16 ͒ are studied using the diffusion quantum Monte Carlo approach. The importance of using accurate guiding wave functions is stressed and we show that the quantum chemical singles-only configuration interaction method and the time-dependent density functional theory within the adiabatic local-density approximation can provide suitable zeroth-order approximations in these systems.Hydrogenated silicon clusters are of significant interest because they may be used to model the absorption and emission of visible light in quantum dots 1 and porous silicon. 2 The optical properties of these systems are strongly influenced by exciton formation, and the accurate prediction of their excitonic gap energies is one of the most challenging theoretical problems in these materials.Excitation energies in hydrogenated silicon clusters have been studied using many theoretical methods, including tight-binding and density-functional methods, and manybody perturbation theory. In this paper we present a study of the excitonic states of small Si n H m clusters using the diffusion quantum Monte Carlo ͑DMC͒ technique. [3][4][5] The small clusters that we study (SiH 4 ,Si 2 H 6 ,Si 5 H 12 , and Si 10 H 16 ͒ are at the molecular limit of semiconductor nanostructures. Small clusters have the advantage that a significant amount of experimental and theoretical data exists with which we can compare our results. Any method capable of giving an accurate description of excitonic behavior in semiconductor clusters at the microscopic level should be able to describe excitons in these clusters, which are therefore important systems for the benchmarking of theoretical methods.DMC is a stochastic method for evolving the imaginarytime Schrödinger equation. The fermionic symmetry is maintained using the fixed-node approximation 6 in which the nodal surface of the wave function is constrained to equal that of a guiding wave function. DMC presents an attractive approach for studying electronic systems because it can achieve very high accuracy and the computational cost scales as N 3 , where N is the number of electrons, which is much more favorable than other correlated wave-function methods.Accurate DMC calculations of ground-state energies have already been demonstrated and in principle such accuracy can also be attained for excited states. The DMC method gives the lowest energy consistent with the nodal surface of the guiding wave function 7,8 and it is therefore crucial that the guiding wave functions give a reasonable description of the excited states of the system. We use guiding wave functions of the Slater-Jastrow type where the Slater part, consisting of a sum of determinants, is multiplied by a nodeless Jastrow correlation factorwhere exp(J) is the Jastrow factor and D i ↑ and D i ↓ are Slater determinants of one-particle up-and down-spin orbitals. The Jastrow factor does not alter the nodal surface of the guiding wave function, which is therefo...

We investigate the efficient construction of guiding wave functions for use in diffusion Monte Carlo calculations of electronic excited states. We test guiding wave functions obtained from singles-only configuration interaction, time-dependent density functional theory, and complete active space self-consistent field methods. The techniques are used to study the first ionization potentials and excited states of silane and methane.

These data suggest that plateau at VO2max is unaffected by O2 availability lending support to the notion of the plateau being dependent on the anaerobic capacity and the classically orientated concept of VO2max.

ABSTRACT:A brief overview of the diffusion quantum Monte Carlo method is given.We illustrate the application to ground-state calculations by a study of the relative stability of carbon clusters near the crossover to fullerene stability, thereby determining the smallest stable fullerene. The application to excited states is illustrated via a study of excitonic states in small hydrogenated silicon clusters.