相对论小核能量一致赝势是流行的相对论效应处理方法, 但是除dhf-nZVPP-2c基组外, 针对此赝势开发基组时都没考虑旋轨耦合效应(SOC), 而dhf-nZVPP-2c的可靠性也主要在密度泛函计算中进行了验证. 本工作采用以标量相对论Hartree-Fock波函数为参考态的SOC-CCSD(T)方法结合此类赝势, 考察5s5p电子及各种基函数对第六周期闭壳层双原子分子性质, 特别是SOC效应的影响. 结果显示, 要可靠计算SOC效应, 在SOC-CCSD(T)计算中要考虑5s5p电子. cc-pVnZ-PP基组和dhf-nZVPP基组不能得到可靠的SOC效应, 而dhf-nZVPP-2c和cc-pwCVnZ-PP基组则能合理描述这些体系的SOC效应. 这两套基组对重元素体系性质的误差与相应基组中基函数数目一致, 顺序为: dhf-TZVPP-2c, cc-pwCVTZ, dhf-QZVPP-2c, cc-pwCVQZ, cc-pwCV5Z. 对这些重元素体系, 要用dhf-QZVPP-2c基组才能得到高精度的键长和谐振频率, 但是对于解离能, 即使cc-pwCVQZ基组仍有一定误差.

Relativistic small-core energy-consistent pseudopotential is a commonly used method to deal with relativistic effects. Unfortunately, spin-orbit coupling effects (SOC) are generally not considered in developing basis sets for this pseudopotential except for the dhf-nZVPP-2c basis sets. Even the dhf-nZVPP-2c basis sets are validated mainly in density functional calculations. In this work, the SOC-CCSD(T) calculations with the scalar-relativistic Hartree-Fock determinant as reference using the small-core energy-consistent pseudopotential are carried out to investigate effects of 5s5p electrons and performance of various basis sets on properties and SOC effects of closed-shell diatomic molecules containing 6 row elements. Our results indicate that it is essential to include 5s5p electrons in SOC-CCSD(T) calculations to provide a reliable description on SOC effects. The cc-pVnZ-PP basis sets and the dhf-nZVPP basis set inadequately capture SOC effects, while the dhf-nZVPP-2c and cc-pwCVnZ-PP basis sets provide reasonable results. The error of these basis sets on properties of heavy-element systems aligns with the number of basis functions in these basis sets and the order is as follows: dhf-TZVPP-2c, cc-pwCVTZ, dhf-QZVPP-2c, cc-pwCVQZ, and cc-pwCV5Z. To obtain accurate bond lengths and harmonic vibrational frequencies of these heavy-element systems, at least the dhf-QZVPP-2c basis set should be employed. On the other hand, error of even the cc-pwCVQZ basis set is still sizeable for dissociation energies.