[DFTB-Plus-User] Geometry Optimization fails

[Sebastian Ehlert]

Hi Jerry,

this is known issue with the 3ob set, for some element combinations the
short range repulsive is just a stub value, like found in the Mg-Cl.skf

    Spline
    24  5.22
    -2.161275291277553e-01  1.0  1.0
    4.05000   4.10000    ... 

This results in an ill-defined repulsive energy at short range, which
will either yield NaN (e.g. for PF4+ or SF5+) or even negative
repulsion energies and nuclear fusion (like in your example), due to
bond lengths shorter than the available spline values. I have a couple
of revised SK files from Tomas Kubar which fix at least the stub
repulsive values for those pairs [1]. However, those have not been
included in the official 3ob set yet.

[1]: https://github.com/dftbparams/3ob/pull/3

Unrelated to the original question, may I ask for the reason for using
GFN2-xTB for preoptimization and than reoptimize just with plain DFTB3?
I would recommend to include at least a dispersion correction like D4

    Dispersion = DFTD4 {
        s6 = 1
        s8 = 0.4727337
        s9 = 0
        a1 = 0.5467502
        a2 = 4.4955068
    }

All the best
Sebastian



On Thu, 2022-03-10 at 20:04 +0000, Jerry Tanoury wrote:
> Dear Forum,
> I am trying to optimize the geometry of the structure in the attached
> xyz file.  I have also attached my dftb_in.hsd input file.  The
> structure has taken from a pre-optimization done with GFN2 using
> Grimme’s XTB software. The problem is that the Mg and Cl atoms crash
> into each other.  The charges for Mg and Cl seem wrong.  Is there
> another set of parameters I should use for this job?
>  
> Thank you for the help,
> Jerry
>  
> Gerald J. Tanoury, Ph.D.
> Principal Scientific Fellow
> Vertex Pharmaceuticals
> 50 Northern Avenue
> Boston, MA 02210
> Office: 617-341-6219
> Cell: 508-740-4986
> jerry_tanoury at vrtx.com
> www.vrtx.com
>  
> 
> 
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