Dear researchers,
When, for example, LanL2DZ basis set is used for a heavy metal such as Ag (Silver), Gaussian divides its electrons to effective core and valence electrons. The question is that what basis is considered to perform such partitioning based on the electron configuration of a given atom? In other words, how can we predict the number of effective core potential and valence electrons based on the electronic configuration of an atom? Is there any trend/unique procedure for this purpose?
Sincerely,
Saeed
The ECP is a MODEL, so you can choose the core and valence electrons any way you like, as long as it's reasonably modelling the experimental findings. The choice of core eletrons in the most commonly used ECPs is completely arbitrary. Lanl2dz has, in most cases, large cores and small valence sets, probably the smallest valence sets that make any sense (only true valence electrons), but other ECPs have smaller cores and larger sets of "valence" electrons.
You may consult the literature on the theory of pseudo-potentials. A concise introduction to the theory of modern, so-called norm-conserving, pseudo-potentials, is presented in the paper by D.R. Hamann, M. Schlüter, and C. Chiang, Phys. Rev. Lett. 43, 1494 (1979). For a more extensive background, you may consult the book Solid State Physics, Volume 24, edited by H. Ehrenreich, et al. (Academic Press, New York, 1970). See in particular the contribution of M.L. Cohen and V. Heine to this volume. Very briefly (see the four requirements enumerated on page 1494 of the above-mentioned paper by Hamann et al.), for a given application it is essential that the core regions of the pseudo-atoms do not overlap. In other words, the distance between any two neighbouring atoms in the system under investigation must be greater than the sum of the core radii of these atoms. The reason for this requirement is that physics inside the core regions of pseudo-atoms is not correctly described.
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.43.1494
Dear Saeedreza,
I think the review written by Dolg is useful to understand how pseudopotenial is constructed in ECP.
http://pubs.acs.org/doi/abs/10.1021/cr2001383
Dear Bartosz, Behnam, and Sirous
Many thanks for your kindness to provide very helpful comments.
Sincerely,
Saeed
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