Applicability of medium-size basis sets in calculations of molecular dynamic polarisabilities

Static and dynamic average polarisabilities and polarisability anisotropies of seven linear non-polar and polar molecules are calculated within the CCS, CC2, and CC3 approximations using a range of medium-sized basis sets: the polarised LPol-<i>n</i> (<i>n</i> = ds, dl, fs, fl), the aug-pc-<i>n</i> (<i>n</i> = 1, 2), the def2-SVPD, and -TZVPD basis sets. Reference values are obtained using a hierarchy of Dunning's (d-)aug-cc-pV<i>X</i>Z (<i>X</i> = D, T, Q, 5) basis sets. The results are discussed together with the available CCSD values in terms of basis set and correlation method errors, and their ratio. Detailed analysis shows that already the def2-SVPD basis set can be used in CCS polarisability calculations. When affordable, the slightly larger aug-pc-1 basis set is recommended, as it leads to significant reduction of basis set error. The def2-TZVPD, LPol-ds, and aug-pc-2 basis sets are optimal choice within the CC2 approximation, with the latter allowing to approach the CC2 basis set limit. The LPol-ds, -dl, and def2-TZVPD sets outperform the aug-cc-pVTZ set in average polarisability CCSD calculations, with the def2-TZVPD being competitive to other reduced-size sets also in determination of polarisability anisotropy. The aug-pc-2 basis is a particularly attractive choice for CCSD, giving the accuracy of aug-cc-pVQZ at a significantly reduced computational cost. The polarisability anisotropy is shown to be more computationally demanding than the average polarisability, in particular with respect to the accuracy of the correlation method and an accurate evaluation of this property requires at least the CCSD model.

本研究采用一系列中等规模基组（basis set），在耦合簇CCS、CC2与CC3近似下，计算了7种线型非极性与极性分子的静态与动态平均极化率（average polarisability）及极化率各向异性（polarisability anisotropy）：涵盖极化LPol-n（n=ds、dl、fs、fl）基组、aug-pc-n（n=1、2）基组、def2-SVPD基组与def2-TZVPD基组。参考值通过邓宁（Dunning）层级化的(d-)aug-cc-pVXZ（X=D、T、Q、5）基组计算得到。本研究结合已发表的CCSD（CCSD）计算结果，从基组误差、关联方法误差及其比值的角度对结果展开讨论。详细分析表明，即便def2-SVPD基组也可用于CCS极化率计算。若计算资源允许，推荐使用规模稍大的aug-pc-1基组，其可显著降低基组误差。def2-TZVPD、LPol-ds与aug-pc-2基组是CC2近似下的最优选择，其中aug-pc-2基组可逼近CC2基组收敛极限。在CCSD平均极化率计算中，LPol-ds、LPol-dl与def2-TZVPD基组的表现优于aug-cc-pVTZ基组；且def2-TZVPD基组在极化率各向异性的计算中，也可与其他缩减规模基组相媲美。对于CCSD计算而言，aug-pc-2基组是极具吸引力的选择，其可在大幅降低计算成本的同时达到aug-cc-pVQZ基组的计算精度。研究表明，极化率各向异性的计算对计算资源的要求高于平均极化率，尤其体现在关联方法的精度方面；且要精准表征该性质，至少需采用CCSD模型。