Coupled Cluster and Quantum Monte-Carlo potential energy curves of the ground state of Bee and Be2+ molecules

Accurate potential energy curves (PECs) have been computed by the Diffusion Quantum Monte-Carlo (DMC) method for the ground state of the Be-2 and Be-2(+) molecules. Multi-determinant trial wave functions (TWF) have been used in the DMC computations. The TWFs have been computed by the Complete Active Space Self-consistent Field (CASSCF) method with the aug-cc-pVTZ basis set. For atomic species (Be and Be+) the full active space and for molecular species (Be-2 and Be-2(+)) the valence full active space have been used. Despite using proper active spaces, the calculated binding energy is not fully compatible with the available experimental value. The source of this discrepancy has been investigated for the present and previous works. Furthermore, the PECs are recomputed by the CCSD(T) level of theory (with all electrons correlated) with the aug-cc-pCVQZ, cc-pV5Z, cc-pCV5Z, cc-pV6Z and 6ZaPa-NR basis sets. In addition, the correlation energy, the dissociation energy (D-e), the ionization potential (IP) and spectroscopic constants have also been computed by the obtained PECs. The Hartree-Fock (HF) energy has been computed by the aforementioned basis sets for all of the studied species. All of the obtained HF energies in this work are more negative than the previous computations, except for the Be atom. (C) 2017 Published by Elsevier B.V.