Gaussian16 Basis Sets: Understanding the Accuracy and Computational Cost

When performing quantum chemical calculations using Gaussian16 on a water molecule, the choice of basis sets plays a crucial role in determining the accuracy of the results and the computational cost involved. In this context, the 3-21G and 6-31G basis sets are commonly used options.

3-21G Basis Set: Minimal Basis Set

The 3-21G basis set is classified as a minimal basis set. This means that it comprises the smallest number of basis functions necessary to describe the electronic structure of a molecule. Despite being minimal, the 3-21G basis set can still provide valuable insights into the molecular properties and interactions of a system.

6-31G Basis Set: Double-Zeta Basis Set

In contrast, the 6-31G basis set belongs to the category of double-zeta basis sets. By employing two basis functions for each atomic orbital, the 6-31G basis set offers increased accuracy and flexibility compared to the minimal 3-21G basis set. This additional level of complexity can help in capturing more nuanced electronic behaviors within the molecule.

It is important to note that neither the 3-21G nor the 6-31G basis set is considered a triple-zeta basis set with polarization and diffuse functions, nor are they quadruple zeta basis sets. Each basis set serves a specific purpose in balancing computational efficiency with accuracy, depending on the requirements of the quantum chemical calculations being conducted.

By understanding the characteristics of different basis sets in Gaussian16 calculations, researchers can make informed decisions on selecting the most suitable option for their specific research objectives. The careful choice of basis sets can significantly impact the reliability and relevance of the computational results obtained.