Freitag, 26. Mai 2017

Reporting Back

Fellow scientists,

it has been quiet here for way over a year now, and I feel I'm owing an explanation: After I was awarded a Feodor-Lynen fellowship early in 2016 (thank you very much for this opportunity, Alexander-von-Humboldt Foundation!), I spent much of the year preparing to move to the other side of the planet to become a Postdoc in Peter Schwerdtfegers research group. On top of that my daughter was born in February, which did the rest to keep me from posting here. But now that we are settled in and out of the worst baby-issues I want to keep this blog alive. For this purpose, I'll first write a couple of posts about articles I've published since my last contribution and eventually about my current projects in the Schwerdtfeger group, namely the development of a polarizable forcefield for Methyl-ammonium lead iodide and the generation of pseudopotentials for super-heavy elements.

Let me begin this series with a project that I have already mentioned in my last three posts: The work on self-consistent, state-specific PCM equilibrium solvation. This self-consistent variant of the SS-PCM approach constitutes a theoretical framework to treat "long-lived" excited states in solution and completes the developments started during my PhD. It allows to equilibrate the excited-state wavefunction with its self-induced polarization, which is the way to go whenerver an excited-states lives for a couple of picoseconds ("long-lived"). Since this is the case for most of the experimentally accessible properties and/or processes originating from excited states, this is a quite important capability. Combined with the perturbation-theoretical variant of the state-specific approach that I've implemented during my PhD, these models enable a calculation and investigation of virtually all photochemical processes in solution, like e.g. ground- and excited state absorption, emission as well as photochemical reactivity. The interface works with all orders and variants of the Algebraic-Diagrammatic Construction (ADC) method developed in Andreas Dreuws group. The article on the topic eventually appeared in PCCP in early 2017. It is available in Q-Chem from version 4.4.2, but documented in the manual starting only from version 5 (release date 1st of June), where you can also find a brief introduction into the theory. It follows a brief summary of the article:

We demonstrated that a general, state-specific PCM in  combination with an ADC(2) or ADC(3) description of the solute’s electronic structure provides excellent energies of solvent-relaxed states and vertical transitions in solution. Since we limited our approach to the state-specific picture, where the solvent effect enters the quantum-chemical calculation only via one-electron charge-density Coulomb integrals, the underlying ADC  equations are unmodified and the model can be used in combination with any flavor of ADC. Moreover, due to this clear separation between quantum-chemical part of the calculation and the solvent model, the results are presumably of general validity for both, the SS-PCM approach as well as the excited-state method.
To validate ADC/SS-PCM, a set of symmetric, ionized dimers was employed, whose lowest energy CT states are formally identical to the broken-symmetry ground state. Computing the latter using the well-established MP/PTE approach and comparing the results to the CT state computed using ADC/SS-PCM, the  deviation between the two methods was found to be 0.02 eV over a wide range of dielectric constants. This holds even for the challenging nitromethane case where electron correlation effects are large.
Ultimately, ADC/SS-PCM was employed to investigate solvent-relaxed potential energy surfaces of 4-(N,N)-dimethylamino-benzonitrile. The agreement with experimental fluorescence data is excellent for the LE state under all circumstances, in particular with ADC(2). For the CT state, however, it was demonstrated that an intra-molecular twisting coordinate has to be considered in detail to achieve a similar agreement. In general, the agreement of ADC(2)/SS-PCM is consistently better than for ADC(3) for fluorescence energies. For the relative energies of the LE and CT states, however, only ADC(3) yields results that are consistent with the experimental observation of dual fluorescence in polar solvents but not in non-polar ones. This was traced back to an underestimation of the energy of the CT state compared to the LE state at second order of perturbation theory.

In my next post, I'll write about an article that originated from a cooperation with the experimental group of Prof. Wagner in Frankfurt, who basically introduced me to chemistry at university level.

So long!