Sunday 5 November 2017

Not all basis sets are created equal

Basis sets are not the most inspiring topic but you can't get around them. That is why we looked at them in our new paper. I am not discussing how many ζ you need or how many diffuse and polarization functions  but I am asking a more subtle question: how big are the differences between basis sets of the same formal type?

This question is addressed in our new paper "Detailed Wave Function Analysis for Multireference Methods: Implementation in the Molcas Program Package and Applications to Tetracene" [full text] that appeared in JCTC. The initial purpose of this paper was to introduce a new toolbox for analyzing multireference computations in the open-source OpenMolcas program package, and I want to encourage people to use this code.

But there is also an important take home message: basis sets of the same formal type (in this case polarized double-ζ) can perform vastly different. And this is not only reflected in the energies but also seen in the densities and overall wavefunctions. In the present case, an atomic natural orbital type basis set had a particularly good performance. This good performance comes at the cost of more primitive basis functions. But these primitive basis functions only play a role in the initial AO integral computations and do not affect the cost of the actual CASSCF/CASPT2 computation at all.

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