Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, A.P. 48-3, Cuernavaca, Morelos 62251, México.
J Chem Phys. 2011 Sep 14;135(10):104311. doi: 10.1063/1.3635403.
The optimized geometries and binding energies for the linear and triangular isomers of the beryllium trimer have been obtained through benchmark multireference averaged quadratic coupled cluster (AQCC) calculations using very large complete active space SCF (CASSCF) references (12 active electrons in 13 and 14 orbitals). Geometries were optimized with the cc-pV5Z basis, while the binding energies (including counterpoise correction) were obtained with the significantly larger aug-cc-pV5Z basis set. The binding energies (27.3 and 16.3 kcal/mol for the equilateral and linear isomers, respectively) are larger than the previous full CI benchmark values, while the corresponding Be-Be equilibrium distances of 4.101 and 4.088 a.u. are smaller. In view of the near-size consistency character of the CASSCF + AQCC method, the fact that all 12 electrons are fully correlated, the active reference space includes 14 orbitals, and the very large basis set used here, we propose to consider these results as reference data for Be(3). Using the electron pair localization function obtained at the CASSCF(12,15) level, it is clearly illustrated that the 2p orbitals lying in the molecular plane play a dominant role in the bonding pattern for the equilateral isomer.
通过使用非常大的完全活性空间自洽场（CASSCF）参考（13 和 14 轨道中的 12 个活性电子）进行基准多参考平均二次耦合簇（AQCC）计算，获得了铍三聚体的线性和三角形异构体的优化几何形状和结合能。使用 cc-pV5Z 基进行了几何优化，而结合能（包括平衡校正）是使用显著更大的 aug-cc-pV5Z 基组获得的。结合能（等边和线性异构体分别为 27.3 和 16.3 kcal/mol）大于以前的全 CI 基准值，而相应的 Be-Be 平衡距离分别为 4.101 和 4.088 a.u. 较小。鉴于 CASSCF + AQCC 方法的近尺寸一致性特征，所有 12 个电子都完全相关，活性参考空间包含 14 个轨道，并且此处使用了非常大的基组，我们建议将这些结果视为 Be(3)的参考数据。使用在 CASSCF(12,15)水平获得的电子对定位函数，可以清楚地说明位于分子平面中的 2p 轨道在等边异构体的键合模式中起主导作用。
