Thursday, June 17, 2010

Spin-state-corrected GTOs

One of the fundamental basic tools in quantum chemistry is formed by the basis set, where Gaussian-type orbitals (GTOs) are fast (because of analytical integrals) but not so good, and Slater-type orbitals (STOs) are excellent but have not all analytical integrals. The latter is not a problem for density functional theory (DFT), because one needs to a numerical integration for the exchange-correlation potential anyway, so one might as well use STOs (as in done in the Amsterdam Density Functional program).

The results obtained depend critically on the quality and size of the basis set, where more is better. This is however not true for spin-state energies as shown by us recently in J. Phys. Chem. A 2008, 112, 6384-6391. There we showed that STOs work fine, already with small basis sets, i.e. they converge fast. Large GTOs give the same result, but the convergence is much slower. And basis sets containing effective core potentials (ECPs) give systematically different (wrong!!) answers, so should not be used for spin-state energies.

Small Pople-type GTO basis sets (like 6-31G* etc.) are in particular very poor. We have studied in this paper (J. Phys. Chem. A 2010, 114, 7191-7197) these basis sets, and showed that their faulty behavior can be largely corrected by increasing the [2d] contraction scheme to a [3d] contraction. In particular what is needed is an additional diffuse d-function. The resulting spin-state-corrected GTO (J. Phys. Chem. A 2010, 114, 7191-7197) basis sets give good results, and are available from the Basis Set Exchange online database.

Tuesday, June 15, 2010

IXth Girona Seminar

In just a couple of weeks (July 5-8) there will be the IXth Girona Seminar, which will deal with in the 2010 version with "Electron Density, Density Matrices and Density Functional Theory". Especially the last part (DFT) will be interesting with many internationally renowned speakers (Evert Jan Baerends, Axel Becke, Stefan Grimme, Gus Scuseria, Weitao Yang to name just a few).

Robin, Talent of the year?

Summer 2015 . My wife's brother came to visit us for holidays, together with their six kids and the grandparents, for the first time s...