xtb currently ships with a preliminary interface to Gaussian. However, the basic interface doesn't include the ability to pass Hessian information, which is critical for some optimization problems (such as finding transition states), and can be used to improve the speed of Gaussian's own ab initio optimizations by reading xtb Hessians as initial Hessian guesses. In addition, the current xtb-gaussian interface is a bit fragile with respect to paths for input-output files.

For work we are currently engaged in, I coded a basic single-file Perl script that addresses these problems (based on an earlier Python version by @kjelljorner and @gabegomes ). It's located here, it's pretty robust and quite fast. It doesn't use any external perl packages so it should just work out of the box.

I currently have it licensed under MIT, meaning you can just integrate it directly in xtb if you want. I considered submitting it as a PR but I wasn't sure what your opinions are on introducing Perl code or on interop with Gaussian. If you are interested, I can probably write the necessary fortran code to move the feature into xtb proper, but I'm not sure when I will have time to do so.

Thanks for the great program!