含旋轨耦合的耦合簇方法中的基组选择

Selection of Basis Sets in Coupled-Cluster Calculations with Spin-Orbit Coupling

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-pw CVQZ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 pseu-dopotential except for the dhf-nZVPP-2c basis sets.Even the dhf-nZVPP-2c basis sets are validated mainly in density func-tional calculations.In this work,the SOC-CCSD(T)calculations with the scalar-relativistic Hartree-Fock determinant as ref-erence using the small-core energy-consistent pseudopotential are carried out to investigate effects of 5s5p electrons and per-formance 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 descrip-tion 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.