Accuracy of Several Wave Function and Density Functional Theory Methods for Description of Noncovalent Interaction of Saturated and Unsaturated Hydrocarbon Dimers

The proper description of noncovalent complexes is a notoriously difficult problem, especially for complexes dominated by the dispersion energy. Accurate and reliable results can be obtained using computationally demanding methods such as the coupled clusters with iterative treatment of single and double excitations and perturbative triples correction (CCSD­(T)), close to the complete basis set (CBS) limit. The sizes of the noncovalent complexes of interest, however, often exceed the computational capability of available computer facilities and software. Computationally efficient yet accurate and reliable theoretical methods are highly desired. In this work, we assembled a small test set of noncovalent complexes of un/saturated a/cyclic hydrocarbon (HC) dimers in order to inspect the accuracy and reliability of several routinely used low-order scaling wave function (WFT) and density functional theory (DFT) methods. The test set comprises dispersion dominated complexes of two different monomer types, saturated and unsaturated. The unsaturated systems are relatively well populated in one of the most popular training data sets for noncovalent complexes, the S22 set of Jurečka et al. The opposite is true for saturated systems, for which rather poor performance of “approximate” methods has been observed. From the results shown is this work, it is clear that unsaturated, e.g., π···π stacked, covalent complexes are described more accurately on average. With the exception of a few “balanced methods”, such as MP2C, MP2.5, SCS-/SCS­(MI)-CCSD, or DFT-D3 with the TPSS and PBE functionals, a simultaneous description of saturated and unsaturated HCs introduces serious errors (i.e., more than 1 kcal/mol).

非共价复合物（noncovalent complexes）的精准描述向来是公认的棘手难题，对于以色散能为主导的复合物而言更是如此。借助计算成本高昂的方法可获得准确可靠的结果，例如采用单双激发迭代处理与微扰三重修正的耦合簇（CCSD(T)）方法，并趋近于完备基组（CBS）极限。然而，所关注的非共价复合物的尺度往往超出现有计算机硬件与软件的计算能力，因此亟需兼具计算效率与准确性、可靠性的理论方法。
本研究构建了一套小型测试集，涵盖饱和/不饱和非环/环烃（HC）二聚体非共价复合物，用以评估数种常用的低标度波函数（WFT）与密度泛函理论（DFT）方法的准确性与可靠性。该测试集包含两类以色散能为主导的复合物，分别对应饱和与不饱和两种单体类型。不饱和体系在非共价复合物最常用的训练数据集之一——尤雷察（Jurečka）等人的S22数据集中占比相对较高；而饱和体系的情况则截然相反，已有研究显示针对这类体系的"近似"方法表现欠佳。
从本研究的结果来看，总体而言不饱和复合物（例如π···π堆叠的复合物）的平均描述精度更高。除少数"平衡型方法"（如MP2C、MP2.5、SCS-/SCS(MI)-CCSD，或采用TPSS与PBE泛函的DFT-D3方法）外，同时对饱和与不饱和烃类复合物进行描述时，会引入显著误差（即误差超过1 kcal/mol）。