QUANTUM MONTE-CARLO STUDY OF BE-2 AND GROUP-12 DIMERS M-2 (M = ZN, CD, HG)

Pure diffusion quantum Monte Carlo calculations have been carried out for Be-2 and the weakly bound group 12 dimers Zn-2, Cd-2 and Hg-2. We have applied relativistic energy-consistent large-core pseudopotential s and corresponding core-polarization potentials for the group 12 atom s. The derived spectroscopic constants (R-e, D-e, omega(e) for Zn-2 an d Cd-2 (Zn-2: 3.88 +/- 0.05 Angstrom, 0.024 +/- 0.007 eV, 25 +/- 2 cm( -1); Cd-2: 4.05 +/- 0.03 Angstrom, 0.031 +/- 0.005 eV, 21 +/- 1 cm(-1) ) are in good agreement with corresponding coupled-cluster results (Zn -2: 4.11 Angstrom, 0.022 eV, 21 cm(-1); Cd-2: 4.23 Angstrom, 0.029 eV, 18 cm(-1)) and available experimental data (Zn-2: 0.034 eV, 26 cm(-1) Cd-2: 0.039 eV, 23 cm(-1)). A comparison with previous results for th e heavier homologue Hg-2 is made. Using a multireference trial wavefun ction for Be-2 we achieved a sufficiently accurate description of the nodes of the wavefunction to obtain a bonding interaction within the f ixed-node approximation, The applicability of this approach has been j ustified in pseudopotential and all-electron calculations. Covalent bo nding contributions which appear in addition to pure van der Waals int eractions for these molecules are analysed in terms of local occupatio n number operators and the associated interatomic charge fluctuations. Static dipole polarizabilities for group 12 atoms and dimers are calc ulated using a differential quantum Monte Carlo method for finite exte rnal electric fields. We have extended this method to pseudopotential calculations by taking into account the electric field dependence of t he localized pseudopotentials. Within the statistical uncertainties ou r results agree with those from coupled-cluster calculations.