Nanoscale Simulations

An accurate electronic structure

An accurate electronic structure for use in AIMD is an important target of my research today. Until very recently, most AIMD has been performed using relatively simple gradient corrected density functionals (GGAs). This is due to the fact that GGAs can be computed relatively efficiently. However, during the ET work, I observed that they can yield qualitatively incorrect results, in particular for certain radicals (such as the biologically highly relevant OH radical) in solution[18,38,43] and for the alignment of the electronic levels across interfaces. This is in line with the experience in the chemical community that GGAs underestimate barriers for chemical reactions. I developed a particularly suitable and reasonably accurate self interaction correction (SIC) functional for use in these systems[18]. This functional has been benchmarked and explored for a number of radical system[18,23,25,35,36,38,43]. Nevertheless, GGAs and their SIC versions do not belong to the most accurate class of functionals, which are hybrid functionals (incorporating exact exchange). In the past, these hybrid functionals have been prohibitively expensive for large scale simulations. However, using an array of computational techniques, we have recently been able to demonstrate that AIMD based on hybrid functionals is feasible[34] and large systems can be computed accurately.[41,42] I believe that hybrid functionals are key for quantitatively accurate AIMD simulations in the future. The availability of wide range of methods of vastly different cost, but also accuracy, is key to multiscale modeling research. Moving the frontiers at all ends of the spectrum is needed to obtain quantitative multiscale models.


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