Feller David
Department of Chemistry, Washington State University, Pullman, Washington 99164-4630, USA.
J Chem Phys. 2016 Jan 7;144(1):014105. doi: 10.1063/1.4939184.
Benchmark quality adiabatic electron affinities for a collection of atoms and small molecules were obtained with the Feller-Peterson-Dixon composite coupled cluster theory method. Prior applications of this method demonstrated its ability to accurately predict atomization energies/heats of formation for more than 170 molecules. In the current work, the 1-particle expansion involved very large correlation consistent basis sets, ranging up to aug-cc-pV9Z (aug-cc-pV10Z for H and H2), with the goal of minimizing the residual basis set truncation error that must otherwise be approximated with extrapolation formulas. The n-particle expansion begins with coupled cluster calculations through iterative single and double excitations plus a quasiperturbative treatment of "connected" triple excitations (CCSD(T)) pushed to the complete basis set limit followed by CCSDT, CCSDTQ, or CCSDTQ5 corrections. Due to the small size of the systems examined here, it was possible in many cases to extend the n-particle expansion to the full configuration interaction wave function limit. Additional, smaller corrections associated with core/valence correlation, scalar relativity, anharmonic zero point vibrational energies, and non-adiabatic effects were also included. The overall root mean square (RMS) deviation was 0.005 eV (0.12 kcal/mol). This level of agreement was comparable to what was found with molecular heats of formation. A 95% confidence level corresponds to roughly twice the RMS value or 0.01 eV. While the atomic electron affinities are known experimentally to high accuracy, the molecular values are less certain. This contributes to the difficulty of gauging the accuracy of the theoretical results. A limited number of electron affinities were determined with the explicitly correlated CCSD(T)-F12b method. After extending the VnZ-F12 orbital basis sets with additional diffuse functions, the F12b method was found to accurately reproduce the best F/F(-) value obtained with standard methods, but it underestimated the CH3/CH3 (-) value by 0.01 eV.
采用费勒 - 彼得森 - 迪克森复合耦合簇理论方法获得了一系列原子和小分子的基准质量绝热电子亲和能。该方法先前的应用表明它能够准确预测170多种分子的原子化能/生成热。在当前工作中，单粒子展开涉及非常大的相关一致基组，范围高达aug-cc-pV9Z（H和H₂为aug-cc-pV10Z），目的是最小化否则必须用外推公式近似的剩余基组截断误差。多粒子展开首先通过迭代单双激发以及对“连接”三激发的准微扰处理（CCSD(T)）进行耦合簇计算，将其推至完全基组极限，随后进行CCSDT、CCSDTQ或CCSDTQ5校正。由于这里研究的体系规模较小，在许多情况下有可能将多粒子展开扩展到完全组态相互作用波函数极限。还包括了与芯/价层相关、标量相对论、非谐零点振动能和非绝热效应相关的额外较小校正。整体均方根（RMS）偏差为0.005 eV（0.12 kcal/mol）。这种一致程度与分子生成热的情况相当。95%置信水平大致对应于RMS值的两倍，即0.01 eV。虽然原子电子亲和能在实验上已知具有高精度，但分子值的确定性较低。这导致难以评估理论结果的准确性。用显式相关的CCSD(T)-F12b方法确定了有限数量的电子亲和能。在用额外的弥散函数扩展VnZ-F12轨道基组后，发现F12b方法能准确重现用标准方法获得的最佳F/F⁻值，但它将CH₃/CH₃⁻值低估了0.01 eV。
