Wednesday, 13 December 2017

Nonadiabatic photodynamics of large systems using the TD-DFTB method

There is a new interface between the Newton-X and DFTB+ packages that allows performing nonadiabatic photodynamics of large systems in a highly efficient manner. This is described in a new paper "Nonadiabatic Dynamics of Cycloparaphenylenes with TD-DFTB Surface Hopping" that just appeared in J. Chem. Theory Comput. Check it out.

You can check Mario Barbatti's blog for some more information.

Wednesday, 6 December 2017

Simulation of ultrafast intersystem crossing using correlated single-reference methods

If you want to do surface hopping dynamics and you do not want to be excessive in your use of computational resources, you have two main options: CASSCF and TDDFT. In the case of CASSCF you have to be really careful when choosing your active space in order to get reasonable results. In the case of TDDFT you have to be really careful when choosing your functional in order to get reasonable results. That is why I am usually not too excited about either one of those methods. On the other hand, I am usually quite impressed by correlated single-reference methods such as CC2 and ADC(2), since they tend to provide good results at reasonable cost and do not require any additional input.

Some time ago we provided an implementation for using ADC(2) as available in Turbomole for surface hopping dynamics in Newton-X. This time, we went a step further and also included the possibility of computing intersystem crossings mediated by spin-orbit coupling using Turbomole with the help of Orca. This new functionality will be available within the next SHARC release (which is coming soon). The paper describing this work just appeared in JCP: "Surface hopping dynamics including intersystem crossing using the algebraic diagrammatic construction method."

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.