# How do I work with Gaussian in Qiskit?

I am trying to run this Qiskit Nature tutorial (Vibrational Structure) and it uses the GaussianForceDriver. Every time I run the Gaussian job from Qiskit I get this message:

QiskitNatureError: Could not locate Gaussian 16 executable 'g16'. Please check that it is installed correctly.


Do I have to install the commercial version of Gaussian, or should it work with just the Qiskit Nature module? If that's the case, what could be the reason for getting this error if I already installed all requirements for Qiskit Nature? Thank you!

## 1 Answer

To use the GaussianForcesDirver in Qiskit, you either need an output file generated by Gaussian (such as this .log file from the tutorial). Or you need an installation (and hence also a license) of Gaussian, as stated in the Qiskit documentation here:

The drivers in the chemistry module obtain their information from classical ab-initio programs or libraries. Several drivers, interfacing to common programs and libraries, are available. To use the driver its dependent program/library must be installed. See the relevant installation instructions below for your program/library that you intend to use.

• Thank you very much! Also, I was wondering if every qiskit nature module has to be compiled with a specific quantum chemistry program. For example, I am doing Electronic Structure Tutorial, but instead of using PySCF (as it is done in the Qiskit tutorial) I want to use Gaussian driver. And I ran an HF job with Gaussian, I have the output ready but when I run my code, it gives me errors (AttributeError: 'WatsonHamiltonian' object has no attribute 'one_body_integrals'). Is it possible to use Gaussian drivers for all modules? Apr 15 '21 at 14:49
• I am not sure I understand your question, but Qiskit Nature can be installed without the classical quantum chemistry programs. Their installation is optional, see the README here. If you have a problem, please ask it in a separate post along with some code snippets and details about the problem. Apr 15 '21 at 16:21