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Atomic species definition
Date: 2007/12/12 12:54
Name: Sumit Saxena

While defining the atomic species, the manual states that "s11 means that one optimized orbitals are constructed from one primitive orbitals for the s-orbital, which means no contraction" I am really unable to follow what the word "contraction" implies. What would it mean for the case as in the manual "C4.5-s2p2d2" beacuse C probably cannot have d orbitals.
Also We have two file in the Directory 'DFT_DATA/VPS' namely C4.0.pao and C4.5.pao. what do those numbers 4.0 and 4.5 mean and how would they effect the calculations.
Sumit Saxena
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Re: Atomic species definition ( No.1 )
Date: 2007/12/12 21:56
Name: Vasilii Artyukhov  <>

C certainly can have d-like PAOs. The PAOs are nothing more than basis functions used to express the density and KS orbitals.

Contraction means that a fixed linear combination is pre-constructed from several primitive functions and used as a single basis function. This allows to decrease the dimension of matrices, while still keeping some of the flexibility of the original (larger) basis set.

The numbers stand for the localization radius (in a.u.) of the PAOs. The smaller the value, the fewer there are non-zero overlap matrix elements, which simplifies solving the equations, but increases the inaccuracy. You should probably start with smaller values and increase them while checking for the convergence of your desired property.
Re: Atomic species definition ( No.2 )
Date: 2007/12/18 20:25
Name: h. jeong  <>

C4.5 - s2p2d2

To understand meanings of "4.5", you should understand concept of Pseudo-Atomic Orbitals.

I suggest the paper of O Sankey, D Niklewski, Physical Review B, 1989 and many papers of SIESTA collaboration and papers of T. Ozaki, H. Kino.

By the standard theory of pseudopotential, s2 means user wants to use two bases of l = 0 characters, nodeless (2s) orbital and one-node (3s) orbital of carbon.

In solids and molecules, carbons are no more isolated atoms with electronic configuration within Hund's rules. Carbons hybridize with their neighbors. To represent well these hybridization in solids and molecules, l=2(d) orbitals of carbons are needed.


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