Interatomic potential for theX1?+g state of Be2

Abstract

m An extended geminal model has been applied to determine the interatomic potential for the X'S: state Be,. By adopting a [lls,9p,6d,4f,2g] contracted Gaussian-type basis, the following potential minimum parameters are obtained: Re = 4.67 a.u. (4.63 a.u.1 and D, = 3.70 mH (3.82 0.05 mH), experimental values in parentheses. A calculation with a nuclei-centered [Ss, 7p, 4d, 2 f, 181 GTO basis plus two sets of bond-type function, each set comprising diffuse (2 s, 2 p , 2 d, 2f, 1 g) GTOs, yielded -3.79 mH as the value of the potential at R = 4.63 a.u. On the basis of an error analysis the best theoretical estimate of the binding energy is determined to be 3.83 k 0.08 mH. The calculated value for the fundamental vibrational frequency is vo

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

“…A more serious challenge for the ®xed-node approximation is the Be 2 molecule which is well-known for the diculties it provides in standard ab initio calculations due to the near-degeneracy of the 2s and 2p atomic orbitals. The deepest potential well has been obtained from ACPF calculations and is even lower than that found by Rùeggen and AlmloÈ f [47]. This is equivalent to performing a complete-active-space self- For the Be 2 molecule we did not include the full CAS spanned by two r g , two r u , one p u and one p g orbitals but rather restricted ourselves to the subset of con®gurations which contribute in the limit of in®nite separation.…”

“…In order to get a trial wavefunction for the Be atom which provides a reasonable description of the nodal structure it is necessary to include at least the 2s 2 and 2p 2 con®gurations [46]. The PP+CPP approach seems to overestimate the attractive interactions, leading to a shorter bond distance of 4.56 bohr compared to 4.67 bohr [47] and 4.627 bohr [19] obtained from all-electron calculations. At the equilibrium distance such a MCSCF wavefunction recovers 98.5% of the CASSCF correlation energy.…”

The ground-state spectroscopic constants of Be2 revisited

“…[1] Intuitively one would expect a purely repulsive potential between two closed-shell singlet atoms -or perhaps a shallow van der Waals-like minimum -and 1 in fact the Hartree-Fock potential is purely repulsive. However, the small (2s) − (2p) gap in atomic beryllium complicates the picture, and when angular correlation is admitted, a tightly bound molecule is in fact found due to an avoided crossing between (2s) 2 + (2s) 2 and (2s) 1 (2p z ) 1 + (2s) 1 (2p z ) 1 curves.…”

Potential energy curve of Be2 in its ground electronic state

“…On the other hand, due to these difficulties, Be 2 is a very suitable test case for critical probing of more sophisticated computational procedures (see e.g., Ref. [1] and references therein). Thus, not surprisingly, Be 2 belongs to the best theoretically studied molecular systems.…”