Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, USA.
J Chem Phys. 2012 Jul 21;137(3):034103. doi: 10.1063/1.4734597.
The basis set convergence of weak interaction energies for dimers of noble gases helium through krypton is studied for six variants of the explicitly correlated, frozen geminal coupled-cluster singles, doubles, and noniterative triples [CCSD(T)-F12] approach: the CCSD(T)-F12a, CCSD(T)-F12b, and CCSD(T)(F12*) methods with scaled and unscaled triples. These dimers were chosen because CCSD(T) complete-basis-set (CBS) limit benchmarks are available for them to a particularly high precision. The dependence of interaction energies on the auxiliary basis sets has been investigated and it was found that the default resolution-of-identity sets cc-pVXZ/JKFIT are far from adequate in this case. Overall, employing the explicitly correlated approach clearly speeds up the basis set convergence of CCSD(T) interaction energies, however, quite surprisingly, the improvement is not as large as the one achieved by a simple addition of bond functions to the orbital basis set. Bond functions substantially improve the CCSD(T)-F12 interaction energies as well. For small and moderate bases with bond functions, the accuracy delivered by the CCSD(T)-F12 approach cannot be matched by conventional CCSD(T). However, the latter method in the largest available bases still delivers the CBS limit to a better precision than CCSD(T)-F12 in the largest bases available for that approach. Our calculations suggest that the primary reason for the limited accuracy of the large-basis CCSD(T)-F12 treatment are the approximations made at the CCSD-F12 level and the non-explicitly correlated treatment of triples. In contrast, the explicitly correlated second-order Mo̸ller-Plesset perturbation theory (MP2-F12) approach is able to pinpoint the complete-basis-set limit MP2 interaction energies of rare gas dimers to a better precision than conventional MP2. Finally, we report and analyze an unexpected failure of the CCSD(T)-F12 method to deliver the core-core and core-valence correlation corrections to interaction energies consistently and accurately.
研究了六种变体的显式关联冻结双子相关耦合簇单双和非迭代三（CCSD（T）-F12）方法，用于研究惰性气体氦到氪的二聚体的弱相互作用能的基组收敛性：CCSD（T）-F12a、CCSD（T）-F12b 和 CCSD（T）（F12*）方法，具有缩放和非缩放三。选择这些二聚体是因为对于它们，CCSD（T）完全基组（CBS）极限基准可用，精度特别高。研究了相互作用能对辅助基组的依赖性，发现在这种情况下，默认的分辨率-身份集 cc-pVXZ/JKFIT 远远不够。总体而言，使用显式相关方法显然可以加快 CCSD（T）相互作用能的基组收敛性，然而，令人惊讶的是，改进的幅度不如仅通过向轨道基组添加键函数来实现的改进幅度大。键函数也大大改善了 CCSD（T）-F12 相互作用能。对于具有键函数的小和中等基组，CCSD（T）-F12 方法提供的精度无法与传统 CCSD（T）相匹配。然而，后者方法在最大可用基组中仍能以比 CCSD（T）-F12 方法在最大可用基组中更好的精度提供 CBS 极限。我们的计算表明，大基组 CCSD（T）-F12 处理的有限精度的主要原因是 CCSD-F12 水平的近似和三的非显式相关处理。相比之下，显式相关的第二阶 Mo̸ller-Plesset 微扰理论（MP2-F12）方法能够以比传统 MP2 更好的精度精确指出稀有气体二聚体的完全基组极限 MP2 相互作用能。最后，我们报告并分析了 CCSD（T）-F12 方法未能一致且准确地提供相互作用能的核-核和核-价相关修正的意外失败。
