Nonrelativistic Effective Core Potentials and Valence Basis Sets

Nonrelativistic Effective Core Potentials and Valence Basis Sets... 

Initially, the level of theory that provides accurate geometries and bond energies of TM compounds, yet allows calculations on medium-sized molecules to be performed with reasonable time and CPU resources, had to be determined. Systematic investigations of effective core potentials (ECPs) with different valence basis sets led us to propose a standard level of theory for calculations on TM elements, namely ECPs with valence basis sets of a DZP quality [9, 10]. The small-core ECPs by Hay and Wadt [11] has been chosen, where the original valence basis sets (55/5/N) were decontracted to (441/2111/N-11) withN = 5,4, and 3, for the first-, second-, and third-row TM elements, respectively. The ECPs of the second and third TM rows include scalar relativistic effects while the first-row ECPs are nonrelativistic [11], For main-group elements, either 6-31G(d) [12-16] all electron basis set or, for the heavier elements, ECPs with equivalent (31/31/1) valence basis sets [17] have been employed. This combination has become our standard basis set II, which is used in a majority of our calculations [18]. 

One more important difference between the GTO and PW approaches is that whereas in the former case the representation of both core and valence orbitals is the same, in the PW formalism the number of PW components needed to correctly describe the behavior of the wave function near the nucleus is prohibitively large. This problem is solved by modeling the core electrons using the pseudopotential approximation, in which it is assumed that the core electrons do not significantly influence the electronic structure and chemistry of atoms. The valence electrons of a particular atom are then considered to move in an effective ionic potential due to the core electrons and the nucleus. Same approach can also be applied to reduce the size of the GTO basis set in calculations without dramatic loss of accuracy. Furthermore, the use of pseudopotentials allows the inclusion of nonrelativistic effects in the calculations, which are particularly important for the chemistry of heavy elements. 

---