Celebrating Martin's PhD defense and wishing him all the best for his postdoc at Northwestern.

]]>**From left:** Daniel (group alumnus), Adam, Carlos, Martin, Kaili, Jonathan, Ben, Kelsie, and Miri.

**Wednesday**

2:00 PM

As soon as The Suspenders arrived and registered, they were taken on a Bus Tour to the National Petascale Computing Facility that houses Blue Waters, the sustained petaflops supercomputer hosted by the UIUC. This is one of the resources that the National Center for Supercomputing Applications (NCSA) offers to the Midwest and National scientific community. As Brett Bode, the Advanced Digital Services Division Director at the NCSA told The Suspenders, Blue Waters is the most powerful supercomputer hosted by a university in the world; it occupies the area of an average soccer field, was built using 81 miles of cabling (approximately the distance between West Lafayette, IN and Urbana-Champaign, IL), and sometimes it might need up to 12 megawatt of power to operate, which is about the same that 20,000 desktop computers need.

5:30 PM

Following the Bus Tour, the poster session featured Daniel Jensen and Martín A. Mosquera.

Daniel’s poster presented Partition Time-dependent Density Functional Theory (PTDDFT), a fragment-based extension of Time-dependent Density Functional Theory (TDDFT) that supports parallel calculations and suggests new ways to compute the time-dependent properties of electronic systems. It also included a simple example showing how useful it is and how it leads naturally to efficient parallel calculations of the time-dependent properties of a system.

Martín’s poster was titled Partition Current Density Functional Theory (PCDFT), a development that shows that the total Time-dependent Current of isolated fragments is enough to determine the electromagnetic potential they are subject to, provided that the initial state is known.

7:30 PM

The UIUC organizers had a Students and Post-docs Mixer planned for the night, where Martín and Carlos showed their skills on several funny games, like building the tallest spaghetti tower that could hold a marshmallow on its top, answering a 40-question trivia, or defying gravity by keeping two balloons on the air at the same time.

After the mixer, the outgoing Suspenders went to Legends, a traditional bar near the UIUC campus where they spent some time talking about many things but hard-science.

**Thursday**

9:30 AM

After breakfast, the invited and contributed talks featured some of the most experienced researchers from the Midwest: Greg Voth, Steve Corcelli, John Herbert, and Arun Yethiraj, who discussed topics ranging from Molecular Dynamics to Electronic Structure methods.

At 11:20 AM the audience was enlightened by our fellow Suspender Jonathan Nafziger, who gave a contributed talk showing that the delocalization and the static correlation errors that undermine the use of Density Functional methods for processes that involve bond breaking can be avoided by using PDFT. He illustrated the use of this new method through calculations on the dissociation of H_{2}^{+} and H_{2}.

7:30 PM

In the evening at the Banquet Dinner, Mark Ratner gave the Keynote Lecture where he talked about global warming, cutting edge materials for solar energy generation, and some of the late history of alcoholic beverages.

After the lecture, professors So Hirata, Anne McCoy, and Arun Yethiraj presented the best posters and presentations. In one of the most memorable moments of the conference, our fellow suspender Martín A. Mosquera’s Partition Current Density Functional Theory was awarded with an Outstanding Poster mention.

**Friday**

9:30 AM

The final day of the conference included the invited presentation of Prof. Chris Cramer on solvation models and Director Brett Bode on the technical and scientific aspects of Blue Waters, as an introduction to the tutorial that took place in the afternoon. When the talks were finished, there was a social hour and box lunches. The Suspenders left Urbana-Champaign soon after that, around 2:00 PM, running away from severe weather conditions.

]]>In an emotional State-of-the-Union speech last week, President Wasserman pointed out that all members of the New Suspenders are united by the *same* partition potential. "*Partition Unites Us*", he said.

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Molecules are made of atoms, yet quantum mechanics, the theory that describes molecules, treats a molecule as a single object. However, when molecules collide at high energies, atoms or molecular fragments are scattered, revealing that there are atoms in a molecule. Thus one may wonder: "What is an atom inside a molecule?" or "How many electrons belong to an atom in a molecule?". A unique answer to this question may be unexistent, but certain criteria can be established to give an answer. In our group we believe the electronic density is all needed to define an atom in a molecule. Density-functional theory (DFT) does, of course, employ such variable to calculate the state of lowest energy of a molecule. DFT despite is exact in principle, needs approximations to the exchange-correlation energy, the missing piece of the puzzle. Most approximations fail to describe molecular dissociation, a process in which an atom (or a fragment) is pulled away from a molecule. For example, we know molecular hydrogen (H_2) dissociates into two hydrogen atoms. Thus, the energy of the dissociated molecule is just twice the energy of a hydrogen atom; many approximations within DFT fails this simple test.

Elliot et al. [1] proposed partition DFT (PDFT), a variation of DFT that, among other features, determines the electronic density of an atom (or fragment) in a molecule and introduces a new way to calculate the ground-state energy of a molecule. In PDFT, an atom, or a fragment, is defined as a Hamiltonian that describes the interaction between the electrons and nucleus of the atom (or nuclei of the fragment), and their kinetic and repulsion energies. The atoms are assumed to be isolated from the rest of the world, but they all "feel" a potential, termed partition potential, whose purpose is to ensure that the electronic densities of the atoms add up to the exact total density of the real system. The partition potential is shown to be related to the residual energy, called partition energy, that the isolated atoms need to match the true energy of the system. Inspired by Elliot et al. [1], we extended PDFT to include electronic spin-densities. An electron spin-density is roughly speaking the number of spin-up or spin-down electrons per volume. We show that the density-functionals of PDFT, i.e., the sum of isolated-fragment energies and the partition energy, are easily extended to the spin-polarized regime. The total spin-density of a molecule is thus partitioned into spin-densities that are localized around their respective fragment nuclei. A variation of any of these densities is equivalent to a variation in the total density. However, a special set of densities that are functionals of the total density exists; this set is employed to suggest a new way to estimate the partition potential. We illustrated that the estimation of the response of the fragment density to perturbations in the total density is important to estimate the partition potential.

Partition spin-DFT (PSDFT) [2], our extension of PDFT, includes a technique to introduce external potentials like magnetic or electric fields. Every fragment in the molecule must be subject to the same external field, allowing us, for example, to employ the partition potential of the molecule in absence of fields as a zero-order approximation to estimate properties like polarizabilities when the external field is weak. The future challenge for our work is to find approximations to the partition potential as a function of the spin-densities, and implement it to simulate 3D molecules. For this sake, the many approximations for the exchange-correlation energy can be used. However, approximations to the kinetic energy are also required, which are usually harder to find. The kinetic effects are not only important to us, but for traditional DFT as well.

References

[1] Partition Density Functional Theory, P. Elliott, K. Burke, M.H. Cohen, and A. Wasserman, Phys. Rev. A 82, 024501 (2010).

[2] Partition density functional theory and its extension to the spin-polarized case, M. Mosquera, and A. Wasserman, Mol. Phys. 111, 505-515 (2013).

]]>A group of 51 researchers at the Pan American Advanced Studies Institute (PASI) on Electronic Properties of Complex Systems in Cartagena, Colombia, discussed major advances in areas such as the use of lasers to create and transfer energy and to provide molecular control. The focus was to find synergy between experiment and theory and build relationships among international participants.

Through a series of discussions and hands-on experiences, the researchers are generating novel strategies to address these problems, which will lead to future research in the fields of molecular electronics and nanoscience.The PASI short course created a cadre of scientists and engineers versed in the electronic properties of complex systems and now primed for international collaborative research in both technology development and applications for current research initiatives.

Since the laser's introduction in the 1960s scientists have developed new application areas for the device including medicine, telecommunications, manufacturing and measurement. Today, the laser is a critical tool for molecular control--a new and important development that seeks to manipulate molecular materials at the atomic level. Fields from chemistry to quantum mechanics use lasers to assist with studies requiring controlled operation.

This PASI award--jointly supported by the NSF and the Department of Energy-was organized by Adam Wasserman at Purdue University. The first meeting took place in June 2011.

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All the matter we see in our lives is composed of atoms. These are made of electrons and nuclei. Knowledge of how electrons behave around the nuclei is all we need to know to manipulate matter at our will in a lab. This idea seems quite pretentious, and many challenges exist that hinders our progress. Scientists in the last century have sought tools and methods to predict how electrons will behave in the presence of nuclei; which ultimately will explain the properties of matter. The laws that rule the electrons' lifes are described by the quantum theory. This offers a set of mathematical equations, quantum mechanics, to predict the outcome of any experiment we perform in a laboratory. However, the solution of those equations is a task of epic proportions. Not even our most modern computers are able to compute what it is required to solve the equations of quantum theory. This has motivated scientists to look for an alternative theory to understand molecules. One of the most used alternative theories is called density-functional theory (DFT). This theory uses the electronic density as its basic variable, which for large systems is the number of electrons per volume. The electronic density is enough to determine the state of lowest energy of a molecule. However, in principle, DFT offers no advantage over traditional quantum mechanics; but its approximations use a basic variable which is much easier to employ for computer calculations.

DFT is widely used by chemists and physicists in their research. However, approximations in DFT are still under development. Many technical challenges exist, and are needed to be solved. Some of these regard describing properly how chemical "bonds" are formed and broken between molecules and the energy changes involved in such processes. Questions like: "How electrons are transferred between molecules?", and "how molecules evolve in time?" are of paramount importance to us, DFT-ists. The purpose of my research is to explore and further develop partition density-functional theory (PDFT), a different version of DFT to study molecules that regards them as composite systems. For example, a molecule can be thought as an object made by smaller objects that are assumed to be isolated from one another (see "Partition Spin Density Functional Theory post"). This theory preserves the density as fundamental variable, but it is regarded as the sum of the smaller components' electronic densities. These densities are an outcome of PDFT and they are the densities that the molecular fragments would have as if they were isolated from one another. PDFT promises to solve problems related to static and dynamic correlation errors of common DFT approximations. Solving these little problems can help DFT to become an even more useful theory than it is nowadays.

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