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.

Wednesday, 5 July 2017

Push-pull chromophores with low singlet-triplet gaps

Had I known how much work it would be, I might have said no when a colleague from organic synthesis asked me to do some calculations for him. But then, the difficult projects are usually also the interesting ones. The question we wanted to answer is why a set of isomeric molecules, that looked very similar on paper, had completely different emission properties and solvatochromic shifts.

First it took quite a while to find an appropriate density functional that could properly reproduce the data. And once I had one, I noticed that the solvatochromic shifts were completely off. That's when I realized that I had to ask someone who actually knows something about solvation. This is where my colleague Jan came into play. He made the smart move of abandoning TDDFT and doing things at the ab initio ADC level. For this purpose, he used a solvation model that he had just implemented. And suddenly everything worked out brilliantly.

For more information, you can find our new paper "Charge transfer states in triazole linked donor-acceptor materials: strong effects of chemical modification and solvation" that just appeared in PCCP.


Friday, 23 June 2017

Excited State Delocalization in DNA

One persistent topic in photobiology has been the question of how many bases are involved in the absorption of a photon of UV light, or in other words how delocalized the absorbing states are. There have been estimates ranging from completely localized monomeric states to delocalization over the whole helix. The question is challenging to study experimentally because the delocalization of the wavefunction is a quantum phenomenon without a direct experimentally observable counterpart. It is difficult to study from a computational point of view because of the extended system size, environmental interactions, and structural disorder. Computational studies using exciton models could not include structural disorder well. Explicit quantum chemistry studies had troubles because of smaller QM regions.

That is why we thought there was need for another more extended study. We used a QM region of eight nucleobases, which could be treated by TDDFT thanks to the GPU based TeraChem code, we did extended sampling by molecular dynamics using the Amber code (also GPU based), and we connected the two by a QM/MM scheme. In total we computed 6000 excited states. To analyze these systematically, we used the TheoDORE code. The results are shown in a new article "Electronic delocalization, charge transfer and hypochromism in the UV absorption spectrum of polyadenine unravelled by multiscale computations and quantitative wavefunction analysis" that just appeared in Chemical Science.


The main results of the study are:
  • Photon absorption occurs predominantly through a collective excitation of two neighboring nucleobases.
  • Full charge transfer (CT) states are only present at higher energies but states with non-neglible CT admixture account for about 50% of the spectral intensity.
  • The experimentally observed hypochromism occurs through perturbed monomer states rather than excitonic or CT interactions.

Wednesday, 24 May 2017

Boron nitride nanoflakes

There is a new paper on metal doped boron nitride nanoflakes written by a colleague from India and myself. In this paper, we investigate what is the best way to represent the excited states in these systems using various quantitative and visual analysis tools. Check it out: "UV absorption in metal decorated boron nitride flakes: a theoretical analysis of excited states" in Mol. Phys. [free full text].


Tuesday, 16 May 2017

Local electron correlation

If you are interested in multireference methods that can be applied to large systems, then you can check out a new paper by us: "Local Electron Correlation Treatment in Extended Multireference Calculations: Effect of Acceptor-Donor Substituents on the Biradical Character of the Polyaromatic Hydrocarbon Heptazethrene" in JCTC. The paper reports a locally correlated implementation for the multireference configuration interaction method. The code is available within the COLUMBUS program package.