The nature of the chemical bond and the role of non-dynamical and dynamical correlation in Be-2

摘要

In the spin-coupled generalized valence bond (SCGVB) description of Be-2, there is a pair of electrons in highly overlapping "inner" orbitals corresponding to a traditional sigma bond, but this bond is compromised by Pauli repulsion arising from its overlap with a second "outer" pair. The presence of this outer pair of electrons leads to a repulsive potential energy curve at long range and a bound, but metastable molecule at short range. To obtain further insights into the nature of the bond in Be-2, we determined the non-dynamical and dynamical correlation contributions to the potential energy curve of Be-2 using four different choices for the zero-order wave function: Restricted Hartree-Fock (RHF), SCGVB, valence-CASSCF(4,4), and valence-CASSCF(4,8). The SCGVB and valence-CASSCF(4,4) wave functions yield similar breakdowns of the total correlation energy, with non-dynamical correlation being the more important contribution. For the RHF and valence-CASSCF(4,8) wave functions, dynamical correlation is critical, without which the potential energy curve is purely repulsive. High accuracy calculations on the HBen-1Be-BeBen-1H molecule as a function of n (n = 1-6) suggest that the intrinsic strength of a Be-Be sigma bond uncompromised by Pauli repulsion is on the order of 62-63 kcal/mol, and its length is 2.13-2.14 angstrom, similar to 60 kcal/mol stronger and similar to 0.35 angstrom shorter than in Be-2.