The two-component calculation of KPACK relies on the use of SOREP. There are two types of SOREPs in use by KPACK: shape-consistent and energy-adjusted SOREP. The former has been developed by the groups of Christiansen and Ermler and their co-workers (EMSL lists their potentials as CRENBL and CRENBS), while the latter by the groups of Dolg and Stoll and their co-workers is often called as the Stuttgart pseudopotential.

Most quantum chemistry programs employ the spin-free scalar-relativistic part of the SOREP known as averaged relativistic effective core potential (AREP) since the formalism is then compatible with nonrelativistic ECP calculation.

In the publications of shape-consistent SOREPs, the given spin-orbit potentials are different from the one that is used in KPACK by some factor. Also, the energy-adjusted SOREPs are published in their original format that are not compatible with KPACK. However, KPACK library contains those SOREPs in the converted format. To minimize the confusion in the use of SOREP, especially with spin-orbit potential, the following naming, which is somewhat based on the nomenclature of Stuttgart pseudopotential,^Dolg13 has been proposed.^Park13

ECPANXYZS

where

A  Authors initials

    CE for Christiansen, Ermler and their co-workers, i.e. shape-consisntent

    DS for Dolg, Stoll and their co-workers, i.e. energy-adjusted

  M for neutral atom

  S for single valence electron ion

  DF for relativistic Dirac-Fock

  WB for quasi-relativistic Wood-Boring

  HF for nonrelativistic Hartree-Fock

  B = low-frequency Breit term included

  Q = QED effect included

S  Spin-free or spin-dependent potential

  SF for AREP

  SO for SOREP

The format of spin-orbit potential follows the Pitzer convention first used in ARGOS program.^Pitzer88 For instance, in case of shape-consistent SOREPs a factor of (2/l) has been multiplied to the published parameters.

Generally, ECPs with A=DS and Y=HF do not come with spin-orbit potentials (hence, S=SF), and Y=WB potentials sometimes provide the spin-orbit potentials but care must be taken before use. Since KPACK assumes the jj-coupling in the SCF stage and thus, the employed SOREPs must qualify a variational usage. Some Y=WB SOREPs are fitted after the construction of ECP and may cause problems within variational treatment. The CE SOREPs and the DS SOREPs with XY=MDF are possible with KPACK in this sense, and the usage is strongly recommended.

The SOREPs can be downloaded from following websites: