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The Beryllium Pentamer: Trailing an Uneven Sequence of Dissociation Energies
Author(s) -
Ascik Peter N.,
Rugango René,
Simmonett Andrew C.,
Compaan Katherine R.,
Schaefer Henry F.
Publication year - 2012
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201101008
Subject(s) - chemistry , trigonal bipyramidal molecular geometry , dissociation (chemistry) , beryllium , atomic physics , bond dissociation energy , basis set , coupled cluster , bond length , crystallography , molecule , computational chemistry , density functional theory , physics , crystal structure , organic chemistry
Recent high‐resolution spectroscopic studies by Merritt, Bondybey, and Heaven ( Science 2009 , 324 , 1548) have heightened the anticipation that small beryllium clusters will soon be observed in the laboratory. Beryllium clusters are important discrete models for the theoretical study of metals. The trigonal bipyramidal Be 5 molecule is studied using high‐level coupled cluster methods. We obtain the optimized geometry, atomization and dissociation energies, and vibrational frequencies. The c∼CCSDT(Q) method is employed to compute the atomization and dissociation energies. In this approach, complete basis set (CBS) extrapolations at the CCSD(T) level of theory are combined with an additive correction for the effect of iterative triple and perturbative quadruple excitations. Harmonic vibrational frequencies are obtained using analytic gradients computed at the CCSD(T) level of theory. We report an atomization energy of 129.6 kcal mol −1 at the trigonal bipyramid global minimum geometry. The Be 5 →Be 4 +Be dissociation energy is predicted to be 39.5 kcal mol −1 . The analogous dissociation energies for the smaller beryllium clusters are 64.0 kcal mol −1 (Be 4 →Be 3 +Be), 24.2 kcal mol −1 (Be 3 →Be 2 +Be), and 2.7 kcal mol −1 (Be 2 →Be+Be). The trigonal bipyramidal Be 5 structure has an equatorial–equatorial bond length of 2.000 Å and an axial–equatorial distance of 2.060 Å. Harmonic frequencies of 730, 611, 456, 583, 488, and 338 cm −1 are obtained at the CCSD(T)/cc‐pCVQZ level of theory. Quadruple excitations are found to make noticeable contributions to the energetics of the pentamer, which exhibits a significant level of static correlation.