Quantum Monte Carlo study of Be 2 and group 12 dimers M 2 (M = Zn, Cd, Hg)

Abstract

Pure diffusion quantum Monte Carlo calculations have been carried out for Be2 and the weakly bound group 12 dimers Zn2, Cd2 and Hg2. We have applied relativistic energy-consistent large-core pseudopotentials and corresponding core-polarization potentials for the group 12 atoms. The derived spectroscopic constants (Re,␣De, ωe for Zn2 and Cd2 (Zn2: 3.88 ± 0.05 , 0.024 ± 0.007 eV, 25±2 cm−1; Cd2: 4.05 ± 0.03 , 0.031 ± 0.005 eV, 21±1 cm−1) are in good agreement with corresponding coupled-cluster results (Zn2: 4.11 , 0.022 eV, 21 cm−1; Cd2: 4.23 , 0.029 eV, 18 cm−1) and available experimental data (Zn2: 0.034 eV, 26 cm−1; Cd2: 0.039 eV, 23 cm−1). A comparison with previous results for the heavier homologue Hg2 is made. Using a multi-reference trial wavefunction for Be2 we achieved a sufficiently accurate description of the nodes of the wavefunction to obtain a bonding interaction within the␣fixed-node approximation. The applicability of this approach has been justified in pseudopotential and all-electron calculations. Covalent bonding contributions which appear in addition to pure van der Waals interactions for these molecules are analysed in terms of local occupation number operators and the associated interatomic charge fluctuations. Static dipole polarizabilities for group 12 atoms and dimers are calculated using a differential quantum Monte Carlo method for finite external electric fields. We have extended this method to pseudopotential calculations by taking into account the electric field dependence of the localized pseudopotentials. Within the statistical uncertainties our results agree with those from coupled-cluster calculations.