QUANTUM MONTE-CARLO DETERMINATION OF ELECTRONIC AND STRUCTURAL-PROPERTIES OF SI-N CLUSTERS (N-LESS-THAN-OR-EQUAL-TO-20)

Variational and fixed-node diffusion Monte Carlo methods are applied to study the structural and valence electronic properties of Sin clusters. Binding energies for n7 agree within 4% with experiments and within 2% when the fixed-node error is decreased by using natural orbitals. For n9 we show that the local density approximation overbinds by 25%. We determined unambiguously (i) the role of correlation in the energy ordering for different structures, including our new lowest energy structure of Si20, and (ii) a different ground state for Si13 - than the one predicted by the local density approximation. © 1995 The American Physical Society.