Perylene radical anion BLYP/6-31+G*//BLYP/6-31G* (a) alpha, and (b) beta electron attachment/detachment densities of the bright 12B3g—>12Au electronic transition.

Martin Head-Gordon and David Chandler, University of California, Berkeley, and Lawrence Berkeley National Laboratory

Research Objectives
Our research centers on the development and application of methods that predict the electronic structure of interesting molecules. We seek to open new classes of chemical problems to study via electronic structure theory.

Computational Approach
Our approach includes electronic structure methods of the density functional theory type, novel density matrix renormalization group calculations of electronic structure, Car-Parinello ab initio molecular dynamics, and transition path sampling methods.

Accomplishments
We have nearly completed a detailed study of the minima and saddle points of the potential energy surface of C₃H₂, corresponding to the reaction between C + acetylene. These results are the highest-level computations yet performed for this system. We have also performed molecular dynamics simulations on the isomerization dynamics of this system.

We have completed two significant time-dependent density functional theory (TDDFT) studies of excited states. The first study was an exploration of excited states of polyenes, in which we showed that the dark excited state, which poses a tremendous challenge for traditional quantum chemical methods, is well treated by TDDFT. This opens the way to applying TDDFT to related systems of biological significance such as carotenoids in the photosynthetic reaction center. The second study was an exploration of the excited states of a novel class of polycyclic aromatic hydrocarbon (PAH) cations which are closed shell. These species have been identified in sooting flames, and may also play a role in interstellar chemistry. However, their visible spectrum has never before been studied either experimentally or theoretically.

The dissociation of a water molecule in liquid water is the fundamental event in acid-base chemistry, determining the pH of water. Because of the short time scales and microscopic length scales involved, the dynamics of this autoionization have not been directly probed by experiment. We revealed the autoionization mechanism by sampling and analyzing ab initio molecular dynamics trajectories. We identified the rare fluctuations in solvation energies that destabilize an oxygen-hydrogen bond.

We have devised a novel importance sampling method for nonequilibrium processes. Using results of this sampling, we demonstrated that statistics of the energy gap between a solute's electronic states are Gaussian throughout the dynamics of nonequilibrium solvation in water. However, these statistics do change in time, reflecting linear response that is nonstationary. Discrepancies observed between the dynamics of nonequilibrium relaxation and of equilibrium fluctuations are thus explained. We analyzed a simple Gaussian field theory that accounts for this nonstationary response.

Significance
Electronic structure theory has emerged as a valuable counterpart to direct experiments for the study of reactive species that may not be easily characterized (if at all) in the laboratory, yet there are still fundamental challenges remaining. It is in these frontiers of electronic structure theory that our research is focused. Our research on chemical and conformational transformations of biomolecules is beginning to yield a novel microscopic picture of biochemical dynamics. Our results may have significant implications for the general understanding of solvent roles in chemical and biochemical processes.

Publications
P. L. Geissler, C. Dellago, D. Chandler, J. Hutter, and M. Parrinello, "Autoionization in liquid water," Science 291, 2121 (2001).

J. L. Weisman, T. J. Lee, and M. Head-Gordon, "Electronic spectra and ionization potentials of a stable class of closed shell polycyclic aromatic hydrocarbon cations," Spectrochimica Acta Part A 57, 931 (2001).

C. P. Hsu, S. Hirata, and M. Head-Gordon, "Excitation energies from time-dependent density functional theory for linear polyene oligomers: Butadiene to decapentaene," Journal of Physical Chemistry A 105, 451 (2001).

http://www.cchem.berkeley.edu/~mhggrp/