Condensed concepts

Sean McConnell, Seth Olsen, and I just finished a paper A Valence-Bond Nonequilibrium Solvation Model for a Twisting Cyanine Dye We study a two-state valence-bond electronic Hamiltonian model of non-equilibrium solvation during the excited-state twisting reaction of monomethine cyanines. These dyes are of interest because of the strong environment-dependent enhancement of their fluorescence quantum yield that results from suppression of competing non-radiative decay via twisted internal charge-transfer (TICT) states. For monomethine cyanines, where the ground state is a superposition of structures with different bond and charge localization, there are two twisting pathways with different charge localization in the excited state. The Hamiltonian designed to be as simple as possible consistent with a few well-enumerated assumptions. It is defined by three parameters and is a function of two  π -bond twisting angle coordinates and a single solvation coordinate. For parameters corres

At the NORDITA water meeting Charusita Chakravarty gave a nice talk that featured the phase diagram below. The figure is taken from a nice Perspective paper in PhysChemChemPhys. Water and water-like liquids: relationships between structure, entropy and mobility  Divya Nayara and Charusita Chakravarty The article gives a nice overview, putting the anomalous properties of water in a broad context, comparing and contrasting to the properties of other liquids for which tetrahedral interactions are dominant. Possible relations between thermodynamics, transport, and structure are also discussed. Key anomalous properties of water [compared to simple isotropic liquids] include -the negative slope of the melting line in the temperature-pressure phase diagram -the temperature of maximum density [277.15 K at 1 atm] -increase in diffusion with increasing density -increase in specific heat, thermal expansion, compressibility upon isobaric supercooling. Water is actually not as u

A more appropriate metaphor is Jacob wrestling with the angel (God). This point was made in a nice talk that Mike Gillan gave last week at the NORDITA water meeting. John Perdew has invoked the metaphor of Jacob's ladder  to describe his "dreams of a final theory" and the quest for an "exact" exchange correlation functional for Density Functional Theory (DFT). Perdew's metaphor was earlier reinvoked by Joost VandeVondele in his talk at the meeting. This painting of Jacob's ladder is by Michael Willmann.  In the Biblical account from Genesis 28 Jacob left Beersheba, and went toward Haran. He came to the place and stayed there that night, ....  And he dreamed , and behold, there was a ladder set up on the earth, and the top of it reached to heaven ; and behold, the angels of God were ascending and descending on it! And behold, the Lord stood above it [or "beside him"] and said, "I am the Lord, the God of Abraham your father and

Sometimes in life we get irritated at people who keep saying the same thing again and again. However, I think if you have an important scientific message you need to realise that you may need to keep repeating it. If you have something original and/or profound to say it is not going to be easy for people to grasp and/or accept. With some talks I have no idea what the speaker is trying to say.  Talks that end with 10 conclusions don't help! Even for good speakers I find I benefit from hearing the talk several times in different forums over a period of time, interspersed with looking at the relevant papers, and sometimes blogging about them. I am not alone. I have noticed that even after someone has heard one of my talks several times in different forums and I have talked informally with them about it, there are basic points they still don't appreciate or get on the third hearing. Action point: don't be shy about giving a similar talk to one you have given before. Try to

I really enjoyed this week's meeting Water: the most anomalous liquid  at NORDITA. This is the first time I have ever been to a workshop or conference that is solely about water. Here are some impressions and a few things I learnt as a newcomer to the field. Just how unique and anomalous is water? Not as unique as I thought. Some other tetrahedral liquids have similar properties. Hydrogen bonding is not what makes water unique Rather it is the tetrahedral character of the intermolecular interactions that arise from hydrogen bonding. This distinction can be seen from the fact that the mW (monatomic water) model captures many of the unusual properties of water. DFT is a nightmare I have written a number of posts that express caution/concern/alarm/skepticism about attempts to use Density functional theory (DFT) to describe properties of complex materials. Trying to use it to calculate properties of a liquid water in thermal equilibrium is particularly adventurous/ambitious

On thursday I am giving a talk "Quantum nuclear effects on hydrogen bonding in water" at the Nordita workshop, "Water: the most anomalous liquid" . Here are the slides.  It is mostly based on this paper.

This week I am in Stockholm at a NORDITA workshop, Water: the most anomalous liquid. I am in a working group on Quantum effects in water. The workshop runs for 4 weeks. There will be about 12 working groups. Each is meant to produce a ten page review that will be then be combined into a review article, co-authored by all the participants. Today we discussed a possible classification of different quantum effects. They are manifested in H/D [hydrogen/deuterium] isotope substitution experiments. For equilibrium properties these isotope effects would be non-existent if the nuclear dynamics is treated classically. This is because at the level of the Born-Oppenheimer approximation the potential energy surface for H and D is identical. For dynamical properties such as the water self-diffusion constant there is a trivial classical effect from the scaling of vibrational frequencies with H/D substitution. As I mentioned before, most quantum nuclear effects are associated with vibrational

How does one stay sane as one struggles to produce talks, lectures, grant applications, and papers? Is there some efficient methodology? I am in the midst of a period of several weeks where I have to give about a dozen talks/lectures. I also have several papers being finished. Hence, this issue is on my mind. Here are a few thoughts that hopefully are helpful to others. I realise that different people have different styles. I think there are several major obstacles to producing material in a timely manner with a minimum of stress: perfectionism, procrastination, loose ends, and distractions . These are somewhat related to one another. First, get started a quickly and early as possible. Just get something down on paper.  A rough outline is a good place to start. Don't censor your thoughts or agonise about details. Just do it! Then start adding material, editing, and polishing. But, spend a limited amount of time . This will mean having the self control to not include that

Jure Kokalj and I just finished a paper, Enhancement of the thermoelectric power by electronic correlations in bad metals: a study of the Kelvin formula  In many strongly correlated electron metals the thermoelectric power has a non-monotonic temperature dependence and values that are orders of magnitude larger than for elemental metals. Kelvin proposed a particularly simple expression for the thermopower in terms of the temperature dependence of the chemical potential. We consider a Hubbard model on an anisotropic triangular lattice at half filling, a minimal effective Hamiltonian for several classes of organic charge transfer salts. The finite temperature Lanczos method is used to calculate the temperature dependence of the thermopower using the Kelvin formula. We find that electronic correlations significantly enhance the magnitude of the thermopower and lead to a non-monotonic temperature dependence. The latter reflects a crossover with increasing temperature from a Fermi liquid

I was saddened to hear of the recent sudden death of Jim Brooks. He is the experimentalist who arguably has had the biggest impact on me scientifically and my career. Jim grew up in Los Alamos in an extended family of physicists. He did a Ph.D at U. Oregon with Russell Donnelly as an advisor, working on low temperature physics. I believe he may have been the first person to put a dilution fridge in a high field [30 tesla] magnet, while working at Boston University and the Bitter Magnet Lab at MIT. This was significant following the discovery of the fractional quantum Hall effect by Tsui and Stormer. After a sabbatical at Princeton with Paul Chaikin [involving the discovery of a quantum Hall state in the field induced spin density wave of a Bechgaard salt ] he began to work almost exclusively on organic charge transfer salts. He made many studies that mapped out their rich phase diagrams [as a function of temperature, pressure, uniaxial stress, magnetic field, and chemical substitut

At the cake meeting this week we discussed Density Matrix Embedding: A Simple Alternative to Dynamical Mean-Field Theory  Gerald Knizia and Garnet Kin-Lic Chan This is an original and promising approach. It is computationally much "cheaper" than DMFT. There are follow up papers that apply the method to quantum chemistry , the problem of defining the QM/MM boundary , the honeycomb lattice Hubbard model , and  calculation of spectral functions . In the latter the bath is frequency dependent. The system is divided into an "impurity" and a "bath". The starting point is the Schmidt decomposition  of the system quantum state. If M is the dimension of the impurity Hilbert space then there are at most M terms in the decomposition. This limits the amount of entanglement between the impurity and the bath. Consider a Hubbard model where the impurity is a single site, then M=4. The reduced Hamiltonian acting on the Schmidt basis states looks like a two-sit

We don't know anything about .... water, high-Tc superconductors, glasses, photosynthesis, enzymes, protein folding .... We don't understand them. No one has any idea how they work. We have no theory. They are unsolved problems. No one can agree on anything.... Sometimes I hear strong claims such as this. I think they are exaggerations. They diminish/ignore/dis-respect significant progress and understanding that has been made.  Unfortunately, these claims of ignorance are often made by people who claim they are going to solve one of these problems, .... once you give them lots of money..... Let me take one specific case: cuprate superconductors. There are many things we know and understand that we did not when they were first discovered. We have a phenomenological theory for all the "macroscopic" phenomena: Ginzburg-Landau! Although not everyone agrees I think it is fair to say that the essential physics is in a one-band Hubbard model and the key physics