[DFTB-Plus-User] diff. bet. dftb+ and gaussian

[Marci]

Dear gxaaas,

You cannot easily compare apples with oranges. DFTB+ is a tight binding
based density functional theory code, while Gaussian is a
hartree-fock/density functional theory code. Furthermore, DFTB+ uses a
minimal basis set while in Gaussian I think you used 6-31g which is "double
zeta" quality, better than the minimal basis set. Thus, even if DFTB+ was a
pure DFT code, it would give a different answer.

This being said, I'm not surprised that the shape of the orbitals are
different. The main question is, what do you want to look at? Are you
interested in single particle orbitals? In this case, you have to be very
careful. Maybe you should look at other properties of the system, such as
equilibrium bond lengths, total energy differences,...

Cheers,
Marton

--
PhD student
Department of Atomic Physics
Budapest University of Technology and Economics

2012/3/1 gxaaas <gxaaas at gmail.com>

> **
>  Hi all,
>      One confusing point needs help. When did the comparison between dftb+
> and Gaussian on frontier orbitals,  I found that  LUMO and LUMO+1 in
> dftb+  have the similar shape to LUMO+1 and LUMO in Gaussian,
> respectively.  Could some ones comment and suggest this point? Thanks!
> The two input files go as follows:
>
>   I:  DFTB+ input:
>  Geometry = GenFormat {
> 3 C
> O H
>      1   1  -2.4890360000E-01  -6.6684060000E-01   0.0000000000E+00
>      2   2   7.1109640000E-01  -6.6684060000E-01   0.0000000000E+00
>      3   2  -5.6935819000E-01   2.3809523000E-01   0.0000000000E+00
> }
>
> Driver = {}
>
> Hamiltonian = DFTB {
>   SCC = Yes
>   SCCTolerance = 1.0E-008
>   MaxSCCIterations = 1000
>   Mixer = Broyden {}
>   MaxAngularMomentum = {
>     O = "p"
>     H = "s"
>   }
>   Filling = Fermi {
>     Temperature [Kelvin] = 0.000000000000
>    }
>   SlaterKosterFiles = {
>     O-O = "O-O.skf"
>     O-H = "O-H.skf"
>     H-H = "H-H.skf"
>     H-O = "H-O.skf"
>   }
> }
>
> Options = {
>   WriteDetailedXML = Yes
>   WriteEigenvectors = Yes
> }
>
> ParserOptions = {
>   ParserVersion = 4
> }
>
>
> II:  Gaussian input,
>
> # b3lyp/6-31g
>
> Title Card Required
>
> 0 1
>  O                 -0.24890360   -0.66684060    0.00000000
>  H                  0.71109640   -0.66684060    0.00000000
>  H                 -0.56935819    0.23809523    0.00000000
>
>
>
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