Statistical physics, molecular modeling and simulation, electronic and photonic materials

David A. Kofke, Professor

Molecular simulation is a method of physical inquiry in which the properties of model materials—defined in terms of their intermolecular interactions only—are studied to “measure” the bulk physical behaviors they exhibit. Our research aims to improve the ability of molecular simulation to do this, while also applying molecular simulation to understanding the behavior of systems that are interesting and of practical importance. Specific research areas targeted by our group are as follows.

Understanding free-energy calculations

The free-energy perturbation (FEP) method is a core technique in computational chemistry. Despite its importance and wide use, it is frequently applied in ways that are inefficient at best, and incorrect at worst. FEP calculations can be performed in either of two directions, depending on which system is taken as the reference. It is well known that these two calculations lead to results that differ systematically. One mistake commonly made is to assume that both calculations are equally wrong, and the best result is obtained by splitting the difference. By applying modeling concepts to the simulation process itself, we have shown that this heuristic is incorrect, and we are formulating a perspective on these calculations that enables them to be applied much more reliably and efficiently. Because free energy is the key to understanding any thermophysical behavior, this work can impact a very broad range of applications. Examples include pharmaceutical formulation and manufacturing, catalysis design, nanotechnology, biochemistry, and many more such fields in which applications of molecular modeling are at the leading edge of research and development.

Evaluation of cluster integrals by molecular simulation

An interesting and exciting extension of our work on free energy methods considers application of molecular simulation to evaluation of so-called “cluster integrals”. These quantities capture the elementary contributions of interactions between two molecules taken alone, three molecules, four, etc. and through a well established theoretical formalism permits them to be combined to yield properties of the bulk phase of 10 23 molecules! A limiting element of this formalism has lied in the inability to evaluate the basic integrals arising in the development. Using ideas from our free-energy work, we have made large advances toward the resolution of this problem. Our larger aim in this work is to permit simulation to yield useful results while focusing on the behaviors of just a few molecules at once. This could be viewed as a powerful way to parallelize the calculations involved in evaluating bulk properties from molecular models.

The above describe our activities to improve the methodology of simulation. In addition we have activities applying molecular simulation to understand complex phenomena.

Effect of association on surface tension

Molecular association, such as that arising from hydrogen bonding, has a profound impact on many physical properties. One property of significant interest is the surface tension, which has plays a crucial role in practical applications. Surface tension influences fluid dynamics, wetting, formation of emulsions and aerosols, and many other fundamental behaviors. We are examining how hydrogen bonding, and molecular association in general, affects the surface tension. As an example of the importance of this effect, we can point to the behavior of hydrogen fluoride (HF). HF is a highly toxic and corrosive material, but has behaviors that make it useful for many industrial processes. An accidental release of HF would be disastrous, as its danger is compounded by the ease with which HF forms a hard-to-disperse aerosol. This aerosol formation is promoted by the very low surface tension exhibited by HF, which in turn is connected to the very strong hydrogen-bonded complexes formed by HF molecules. Understanding of these phenomena can lead to additives and other measures that can improve the safety of HF processing.

Miscibility in compound semiconductors

The great advances we all see in electronics and optical devices owes very much to the engineering of advanced materials. Much attention recently has been given to the development of short-wavelength (blue) lasers, that can greatly increase the ability of optical media to store data. Compound semiconductors are key materials for the construction of such devices, and the processing of these materials requires a good understanding of their physical behaviors, including their thermodynamic properties. Of particular interest is solid-phase immiscibility, which tends to cause these systems to forms domains rich in one or the other element. The state of the art in modeling of this behavior begins from a molecular model, but then employs a rather crude thermodynamic model to extract the miscibility behavior. We are instead applying molecular simulation to the same molecular model to extract the true miscibility behavior of the model system, as well as more detailed information about its clustering. This should give a more certain and correct characterization of the way these systems separate, and enable improved design and manufacturing of devices based on them.

Object-oriented programming in molecular simulation

This is less a research than a development project driven by an interest in applying molecular simulation as a teaching tool. We are investigating the feasibility of using molecular simulation teaching tools as a basis for constructing extensible, object-oriented research-quality molecular simulation codes. Etomica is the name of the API and development environment that we are constructing. We now have a page devoted to describing and disseminating it: www.ccr.buffalo.edu/etomica.