The Beryllium tetramer: Profiling an elusive molecule

摘要

The structure and energetics of Be_4 are investigated using state-of-the-art coupled-cluster methods. We compute the optimized bond length, dissociation energy, and anharmonic vibrational frequencies. A composite approach is employed, starting from coupled-cluster theory with single, double, and perturbative triple excitations extrapolated to the complete basis set (CBS) limit using Dunnings correlation consistent cc-pCVQZ and cc-pCV5Z basis sets. A correction for full triple and connected quadruple excitations in the smaller cc-pCVDZ basis set is then added, yielding an approximation to CCSDT(Q)/CBS denoted c～CCSDT(Q). Corrections are included for relativistic and non-Born-Oppenheimer effects. We obtain D_e = 89.7 kcal mol ~(-1), D_0 = 84.9 kcal mol~(-1), and re = 2.043. Second-order vibrational perturbation theory (VPT2) is applied to a full quartic force field computed at the c～CCSDT(Q) level of theory, yielding B_e = 0.448 cm-1 and fundamental frequencies of 666 (a _1), 468 (e), and 571 (t_2) cm~(-1). Computations on the spectroscopically characterized Be2 molecule are reported for the purpose of benchmarking our methods. Perturbative estimates of the effect of quadruple excitations are found to be essential to computing accurate parameters for Be_2; however, they seem to exert a much smaller influence on the structure and energetics of Be_4. Our extensive characterization of the Be_4 bonding potential energy surface should aid in the experimental identification of this thermodynamically viable but elusive molecule.