Semicore ECPs

Generation of the REPs is perhaps the most critical step in the derivation of an ECP/valence basis set scheme. The major question is What core size to use The choice of orbitals to include in the core is fraught with uncertainty. One needs to strike a balance between chemical accuracy and the desire to replace as many core electrons as possible. Replacement of all core electrons by the potential (full-core ECPs) is most prevalent for p-block elements, but not replacing the outermost core electrons (semicore ECPs) is the norm for d- and f-block metals. -i° This issue is discussed in detail in the survey of ECP applications later in this chapter. 

Large-core pseudopotentials are well known to lead to sizable errors [122-125] we thus recommend the use of small-core ECP s that include semicore orbitals in the valence space [120],... 

The basis sets used for transition metal systems should be of at least double-zeta quality, and commonly larger sets are used it is especially common to have the d functions more flexibly contracted, The use of effective core potentials (ECPs) can reduce the computational effort significantly and allow inclusion of relativistic effects. Experience has shown that if the system contains significant M" " character (where M represents a transition metal atom) it can be important to include the semicore electrons (i.e., the 3s and 3p orbitals for the first transition row) in the valence treatment and not include them in the ECP. While relativistic effects such as the core contraction can be included in the ECP, it is perhaps not surprising that for the third transition row, one must go to treatments that include additional relativistic effects, such as spin-orbit coupling. 

---