Background:

• Arnold and Mabel Beckman Young Investigator Award (2006)
• Indiana University Summer Faculty Fellowship (2005)
• Camille and Henry Dreyfus New Faculty Award (2003)
• Postdoctoral Associate, University of Utah, Salt Lake City, Utah, 2003
• Postdoctoral Associate, Rice University, Houston, Texas, 1999

Our research efforts are on the interface of chemistry, computational physics and applied mathematics. We deal with the development of new theoretical methods and the subsequent implementation of these into efficient computational models. The methods are derived with an aim to help solve problems in biophysical chemistry, atmospheric chemistry and the area of nano-material science.

Ab initio quantum dynamics for large systems: We have recently developed an efficient computational approach to allow the simultaneous dynamical treatment of electrons and nuclei. This approach is called Quantum Wavepacket Ab Initio Molecular Dynamics (QWAIMD). The approach allows a massively parallel implementation; computational treatment of simultaneous dynamics of electrons and nuclei in medium sized chemical systems can now be treated over 100s of computer processors leading to an efficient computational methodology. Furthermore, our approach allows the flexibility to treat a subset of the nuclei in a quantum mechanical fashion while simultaneously studying the dynamical evolution of the electrons with the majority of nuclei treated in a classical fashion. (See for example J. Chem. Phys.122, 114105 (2005) for details on the theoretical aspects.) Why would one want to treat "some nuclei using quantum mechanics"? It turns out that this problem is of relevance in many biological enzyme problems and also in atmospheric chemical problems, the study of which we are now actively pursuing. As an example, of our current study, the figure on the left represents the active site of the enzyme "lipoxygenase" where the quantum dynamical nature of a transferring hydrogen atom dictates the kinetics. Such interesting chemistry is also found in many atmospheric chemical problems as a result we are currently applying this methodology to problems in biological chemistry and atmospheric chemistry. In addition, for problems where a quantum mechanical treatment of nuclei is not necessary, as part of a collaborative effort we have invented another powerful new ab initio molecular dynamics approach that enables the study of chemical dynamics in medium-to-large size systems. Systems with more than 100 atoms can be routinely handled within this scheme at an accurate level of theory. The approach is based on the simultaneous dynamics of the electronic degrees of freedom along with the nuclear degrees of freedom (treated classically). The method is called Atom-centered Density Matrix Propagation (ADMP). (See for example J. Chem. Phys.115, 10291 (2001) for details on the theoretical aspects.

We are also currently modifying and generalizing the Quantum Wavepacket Ab Initio Molecular Dynamics (QWAIMD) approach described above to treat the transport ot excess electrons and flux of electron densities through molecular wires and other nano-scale devices. Key theoretical challenges remain here which we are currently in the process of surmounting. Some of the important issues that we are currently focusing our attention on here, include the accurate description of an electronic flux through a molecular wire subjected to open system boundary conditions.

The QWAIMD also presents many interesting problems from a scientific computing point of view which allows graduate students and post-doc in our group to become familiar with state of the art ideas in numerical analysis.

Another active area of research in our group involves the use of ab initio molecular dynamics (with and without nuclear quantum effects) to study the spectroscopic properties of small molecular clusters. As part of a team of collaborators we have recently noted that the hydrated proton in a water cluster has an amphiphillic (hydrophobic hydrophillic) character. These results were obtained originally from ab initio and empirical molecular dynamics and later confirmed by experiment. These will lead to profound implications in biological and atmospheric problems where proton transfer plays a critical role. We have also shown that dynamics can play a critical role in determining the vibrational spectra of such small "in-flux" clusters.

Electronically non-adiabatic dynamics in biological systems: Another aspect that we are actively pursuing is the study of non-adiabatic chemical processes in complex. In many systems a significant portion of the electronic dynamics can involve multiple electronic states. For example, such is the case for the protonated-Schiff base, Retinal, which constitutes an important component of vision. The figure to the left depicts the active site here, i.e., the retinal (in yellow) surrounded by amino acids that are in close proximity. We are working on extensions to our ab initio dynamics formalisms that will allow the study of such challenging chemical, biological and biophysical problems.

Non-perturbative treatment of interaction of radiation with matter: We are also pursuing the development of rigorous computational and theoretical methods that will allow the non-perturbative treatment of interaction of radiation with matter to study non-linear optical properties and the interaction of intense laser fields with matter in the near-field limit.

Finally, while deriving new computational and theoretical methods, we constantly remind ourselves of the famous statement from the best known theoretical scientist of modern history:

The scientific theorist is not to be envied. For nature, or more precisely experiment, is an inexorable and not very friendly judge of his work. It never says "YES" to a theory. In most favorable cases it says "MAYBE", and in the great majority of cases simply "NO"... Probably every theory will some day experience its "NO" - most theories, soon after conception. — ALBERT EINSTEIN, NOV 11, 1922

Selected Publications:

S. S.Iyengar and J. Jakowski, "Quantum WavepacketAb Initio Molecular Dynamics: An approach to study quantumdynamics in large systems". J. Chem. Phys. 122 114105 (2005).

S. S. Iyengar,"Ab Initio Dynamics with wave-packets and density matrices". Theo.Chem. Accts. Special issue on "NewPerspectives in Theoretical Chemistry" In Press. (URL)

S. S. Iyengar andM. J. Frisch,"Effect of time-dependent basis functions and their superpositionerror on Atom-centered Density Matrix Propagation (ADMP):Connections to Wavelet theory of multi-resolution analysis". J.Chem. Phys.121, 5061 (2004)

S. S. Iyengar,"Dynamical effects on vibrational and electronic spectra of HO₂water clusters: an ab initio molecular dynamics study". J. Chem.Phys. 123 , 084310 (2005)

S. S.Iyengar, M. K. Petersen, C. J. Burnham, T. J. F. Day, V. E. Teige andG. A. Voth,"The Properties of Ion-Water Clusters. I. The Protonated 21-WaterCluster". J. Chem. Phys. 123 , 084309 (2005)

S. S.Iyengar, T. J. F. Day, and G. A. Voth, "On the Amphiphilic Behavior ofthe Hydrated Proton: An Ab Initio Molecular Dynamics Study". Int.J. Mass Spectrometry 241,197-204(2005).

M.K. Petersen, S. S. Iyengar, C.J.Burnham, T. J. F. Day, and G. A. Voth, "The Hydrated Proton at theWater Liquid/Vapor Interface". J. Phys. Chem. B 108 ,14804 (2004).

N. Rega,S. S. Iyengar, G. A. Voth, H. B. Schlegel, T. Vreven and M. J. Frisch,"Hybrid Ab-Initio/Empirical Molecular Dynamics: Combining the ONIOMScheme with the Atom-centered Density Matrix Propagation (ADMP)Approach". J. Phys. Chem. B 108 4210-4220(2004).

S. S. Iyengar, H. B. Schlegel, J. M. Millam, G. A. Voth,G. E. Scuseria, M. J. Frisch, "Ab Initio Molecular Dynamics:Propagating the Density Matrix with Gaussian orbitals. II.Generalizations based on Mass-weighting, Idempotency, EnergyConservation and Choice of Initial Conditions". J. Chem. Phys. 115, 10291 (2001).

S. S. Iyengar, D. J. Kouri, D. K. Hoffman, "Particularand Homogeneous Solutions of Time-Independent Wavepacket SchrodingerEquations: Calculations Using a Subset of Eigenstates of Undamped orDamped Hamiltonians". Theoretical Chemistry Accounts. 104(2000) 471-483.

S. S. Iyengar, G. E. Scuseria, A. Savin, "Bounding theExtrapolated Correlation Energy Using Pade Approximants". Int. J.Quant. Chem. 79 (2000) 222-234

S. S. Iyengar, D. J. Kouri, G. A. Parker, D. K. Hoffman,"Estimating the Upper and Lower Bounds for the Eigenvalues of anymatrix". Theoretical Chemistry Accounts. 103(2000) 507-517.

S. S. Iyengar, G. A. Parker, D. J. Kouri, D. K. Hoffman,"Symmetry-Adapted Distributed Approximating Functionals: Theory andapplication to the ro-vibrational states of H3 ". J. Chem. Phys. 110 , 10283 (1999).