The focus of the current flagship project on Car-Parrinello methods in chemistry has been on development and application of methods for first principle computation of vibrational and electronic spectra of condensed molecular systems. An extensive study the infra red spectrum of aqueous uracil has been completed[1]. The good agreement with experiment made it possible to exploit the microscopic data on structure and dynamics provided by the molecular dynamics simulation in a detailed analysis of the substantial changes in spectral band shapes due to hydrogen bonding to the solvent.

Solvent fluctuations were also shown to play a crucial role in the first results obtained by the PDRA, Leonardo Bernasconi, on electronic absorption spectra of aqueous solutes using time dependent density functional methods (at the GGA/ALDA level)[2]. The crucial feature distinguishing our approach from the reaction field or QM/MM calculations available in the literature is that solvent and solute molecules are treated on equal footing. As an application we studied the UV spectrum of aqueous CuI and AgI ion [3]. Whereas the AgI can still be interpreted in terms of crystal field splitting in an average solvent field, this is not possible for the CuI spectrum. Absorption by CuI is dominated by the large dynamical rearrangements of its solvent shell, making the spectrum essentially a superposition of several different optically active configurations. An example of such a configuration is shown in the figure.

 
 
Electronic absorption spectrum of aqueous transition metal ions as obtained using ab initio molecular dynamics methods implemented using the CPMD code. Comparison of the CuI and AgI spectrum (left) and a sample configuration of the CuI ion and surrounding solvent (right). The solvent shell is subject to large thermal rearrangement with a preference for twofold coordination as indicated by the bonds.

However, as noted in our test calculation on aqueous acetone[2], The "global" GGA/TDDFT scheme also introduces certain complications in the form of spurious charge transfers between solute and solvent[2].We are currently implementing hybrid (exact exchange) DFT methods (B3LYP, PBE0) in our plane wave code (CPMD). The first results indicate that this approach, while computationally expensive, suppresses the false charge transfers. Other projects currently in progress are collaborations with the Manchester group (Jitariu/Hillier) on supercritical aqueous chemistry and the Bath group (Ruggiero/Williams) on aqueous chemistry of a model organic molecule (lactone).

References

[1] Ab initio molecular dynamics computation of the infrared spectrum of aqueous uracil, M.-P. Gaigeot and M. Sprik, J. Phys. Chem. B 107 (feature article), 10344, (2003).

[2] Time dependent density functional theory study of charge-transfer and intra-molecular electronic excitations in acetone water systems, L. Bernasconi, M. Sprik, J. Hütter, J. Chem. Phys 119 (Dec. 2003).

[3] Coordination environment and optical response of Cu+ and Ag+ in aqueous solution: A DFT/TDDFT study, L. Bernasconi, J. Blumberger, M. Sprik and R. Vuilleumier (in preparation).