Quantum Monte Carlo characterization of small Cu-doped silicon clusters: CuSi4 and CuSi6

The relative energies, binding energies, and adsorption energies of three C uSi4 and two CuSi6 clusters have been computed in the fixed-node diffusion Monte Carlo (FNDMC), CASSCF, and B3LYP DFT methods. These results are compa red with the earlier Hartree-Fock (HF) and B3LYP DFT investigations of thes e systems by two of us [C. Xiao and F. Hagelberg, J. Mol. Struct.: THEOCHEM 529, 241 (2000)]. The very close energy level spacing of the isomers under consideration confirms the previous work of Xiao and Hagelberg. The FNDMC results show some qualitative discrepancies with B3LYP DFT, and HF findings . They also confirm the appropriateness of the B3LYP DFT method for the pre diction of the most stable CuSi4 isomer, while the CASSCF method compares m ore favorably with FNDMC for adsorption energies than B3LYP DFT. (C) 2001 A merican Institute of Physics.