This is a wrapper that allows xtb to be used as part of Gaussian. This is useful in particular for TS optimizations that xtb doesn’t otherwise do, and that Gaussian does very well.

As xtb already has most of the functionality built-in, we mostly just leverage that. The only improvement really is the use of some perl scripting to transform xtb Hessians to Gaussian Hessians, which xtb doesn’t do on it’s own. The short script is in xtb-gaussian.

The files examples/optimize.com and example/QST2.com provide examples of usage (the optimization of a water molecule and finding an SN2 transition state and reaction path using quasi-Newton synchronous transit). In short, the Gaussian input file should have a method line like this one,

%chk=ts
# external="xtb-gaussian -P 12 --etemp 300.0"
  opt=(calcfc,ts,noeigentest,nodownhill,nomicro)

Here, we set the number of threads for xtb to 12 (-P 12 above) and perform a transition state search using an initial xtb Hessian and xtb gradient calculations. Additional arguments to xtb can be passed in the external variable (like --etemp 300.0 above). Passing --log-all as the first argument to xtb-gaussian will mix in the (often very long) xtb output with the Gaussian output, which is not done by default for performance and easier parsing of Gaussian log files.

Note that the molecular charge included in the Gaussian molecule deck is automatically used the wrapper script. However, that is not true of the multiplicity! The script will warn of non-singlet multiplicity, which you can technically treat by passing the appropriate --uhf argument to xtb in the external string (caveat emptor).

Installation consists solely of putting xtb-gaussian and xtb both on $PATH so that they are visible to Gaussian. The only dependency is Perl. This code was tested against xtb 6.2.3 and xtb 6.3.1rc1 binaries on linux.

There are some things to keep in mind to ensure the best possible performance, mostly because of Gaussian is a bit old-school.

Firstly, passing %CPU to gaussian makes xtb use 1 thread. So don’t do it! Furthermore, %NProcShared= will make Gaussian wait busily for xtb using a full half of the NProcShared cores while xtb is running, but use all the cores for it’s own routines (like Hessian diagonalization). It is best to give only 1 or 2 processes to Gaussian and the rest to xtb using -P as above if Hessians are often computed (with xtb parallelization) or the system is large. Otherwise, allocating some threads to Gaussian can make the code faster.

Secondly, performance is very IO centric as Gaussian and xtb keep writing and reading files over and over again. The wrapper respects the Gaussian scratch directory. Running with $GAUSS_SCRDIR pointing at a fast filesystem (such as /dev/shm/ on linux) makes a huge difference.

I wrote this perl version based on a Python script developed by postdoctoral fellows Kjell Jorner (at AstraZeneca, UK) and Gabriel dos Passos Gomes (University of Toronto, Canada).