Thursday, 19 November 2015


When it comes to citation indices it is better to be a small fish in a big pond rather than a big fish in a small pond. But that would of course be only a very technical reason for joining a big collaboration, and indeed I have enjoyed working with the MOLCAS community a lot. So, aside from the fact that I feel like I am tweaking my citation stats a little bit, I am very happy that the new MOLCAS 8 paper just came out in J. Comp. Chem.

My work was related to the interface to the multireference configuration interaction code COLUMBUS. It is now possible to mix the different modules of MOLCAS and COLUMBUS to get the best out of both programs. While T. Müller was the one actually doing the hard stuff, my task was to work on the little things and the user friendliness.

My next plan is to interface our wavefunction analysis library libwfa to MOLCAS. Currently, we have this only in Q-Chem for singlereference methods but it would be nice to extend the capabilities to the multireference world. The basic functionality like natural transition orbitals or the charge transfer number analysis is indeed already available for MOLCAS and COLUMBUS through an interface to TheoDORE. But the most fun stuff is still missing. And it is easier to supply such extended functionality through an integrated library rather than an external program.

Friday, 6 November 2015

Correlation and Charge Transfer in TDDFT

The charge transfer problem of time-dependent density functional theory (TDDFT) is well known. TDDFT also has a problem for extended π-systems even if there is no net charge transfer. Is there a connection? Yes, correlation!

In a recent communication in J. Chem. Phys., Exciton analysis in time-dependent density functional theory: How functionals shape excited-state characters,  we discuss the relation between static and dynamic charge transfer effects and their description in TDDFT. It is shown that both kinds of charge transfer are detrimental to local functionals while the problems can be remedied through Hartree-Fock exchange. The results are interpreted in terms of excitonic correlation. Only the inclusion of Hartree-Fock exchange leads to bound excitons while effective electron-hole repulsion is obtained for local functionals.

Tuesday, 7 July 2015


Electron correlation has always been this intangible thing to me. You can talk about correlation energy, about multi-configurational wavefunctions, about differences between dynamic and static correlation. But what is actually going on? Let's tackle the problem with statistics! This is what we did in our new paper Statistical analysis of electronic excitation processes: Spatial location, compactness, charge transfer, and electron-hole correlation in J. Comp. Chem.

How do you quantify correlation? With a correlation coefficient! All you need is a function in two variables, then you can compute its covariance and normalize it by the standard deviations. A logical choice for such a function would be the 2-body density and given enough time and/or people doing it for me, I will look at that. For now we chose the 1-particle transition density matrix (1TDM) between the ground and excited state. This function describes the electron and hole quasi-particles in the exciton picture (see this post). And, among other things, we can compute the correlation coefficient between the electron and hole.

A good way to understand this new tool is in the case of symmetric dimers. Because of the symmetry all orbitals and states in such a system are delocalized over the whole system and no net charge transfer can be seen. But it is clear that the charge transfer states do not disappear: They are just arranged in symmetric linear combinations yielding the charge resonance states. On the other hand the local excitations are arranged in excitonic resonance states. Applying the new tool to this type of system shows that the difference is a correlation effect: Positive correlation yields bound excitonic states while negative correlation represents charge resonance

Friday, 19 June 2015

Color Charges

Should I boycott journals that charge money for color figures? Or at least send them only the papers that were rejected somewhere else? For a paper I would usually spend months researching a topic and probably another few weeks preparing graphics that allow a quick comprehension even for the hectic reader. What if during this process I find out that the best way to represent my results is by using a few colored lines? How can a journal editor in their right mind refuse to print those colored lines? It is not only out of respect for my work but also not to waste the time of any readers trying to decipher the greyscale figures that it looks like a clear decision to me.

The absurd thing is that color charges are usually given per image. You pay the same price for a full page color photo as for those one or two colored lines making a graph so much more comprehensible. I can see why a journal would not want to pay for the former. But I am sure it would be possible to add a few colored lines at an acceptable cost.

To answer the question from above: I am not going to boycott any journals. But the question of whether or not I agree with the publication process, certainly plays a role. Usually I have to choose a journal with the words Phys and Chem in it, arranged in arbitrary order and multiplicity. I could not tell you, which one has a higher impact factor. But I can tell you whether or not I like their publication policies. If you are interested, my favorite is J. Chem. Phys.: easy to use manuscript template, free color figures, reasonable copyright policies, and no nonsense.

Thursday, 21 May 2015

Twisted Intramolecular Charge Transfer

Admittedly, we are not the only people working on Dimethylaminobenzonitrile (DMABN) and its dual fluoresence. But it is an interesting system worth looking at. Our paper about this topic "Intramolecular Charge Transfer Excited State Processes in 4-(N,N-Dimethylamino)benzonitrile: The Role of Twisting and the πσ* State" is finally released after starting this project about four years ago. Check it out if you are interested.