[DFTB-Plus-User] basis set

[Bálint Aradi]

Dear Jacek,

> 1)   Why do we have  more than one exponent for hydrogen?   This looks
> like contracted gaussian  fitted to Slater orbitals.
>   The above numbers in the basis set description  do not seem  to be
> a Slater type basis set, but rather STO-nG type basis set.
>  Am I correct?  Is it really Slater basis set  or STO-nG basis set
> used to parametrize  DFTB integrals?

  Every basis function is a linear combination of Slater-functions with
different exponents.

> 
> 2) If the  DFTB  basis set is  indeed STO-nG then:

 No, there are no Gaussian functions involved, those are really STOs.

> 3) Can anyone explain this the basis set format to  me?
> I would like to use this basis set  (or any other info) to write a
> small program that can  calculate matrix of   dipole moment integrals.

 The best place to look at it is the slater.F90 file (subroutine
SlaterOrbital_getValue_explicit) of Waveplot, where you see, how the
value of the radial part is calculated, using the exponents and the
coefficients, which are read from the file you mentioned. Please note,
that the exponents are automatically multiplied by -1 when read in,
therefore the algorithm does not contain this minus any more.

  Best regards

  Bálint

-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/en/~b_aradi


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