Water in general, as well as its role in hydration processes, is of outmost importance in many areas of chemistry, physics and biology. Despite of the ubiquitous presence of aqueous solutions, many fundamental aspects are not yet fully understood and/or cannot yet be simulated with sufficient accuracy. Our work focuses on simulations of charge transfer and/or unusual solutes (such as the solvated electron) and/or confined water where conventional molecular dynamics simulation techniques based on (semi-)empirical force fields are of questionable accuracy or even impossible. Instead, we tackle these questions using mixed quantum/classical (Born-Oppenheimer) molecular dynamics simulation methods where energies and forces are calculated for every time step from "first principles/ ab initio" or density functional (DFT) calculations of the electronic structure. In most of our work we systematically investigate the effects of hydration by comparing systems in the gas phase, in clusters with increasing number of solvent particles, finally approaching the bulk solvent limit.
See also our 2011, 2015 press releases
Ion pairing in water
Burkhard Schmidt with Eva Pluharova
Cooperation with Ondrej Maršálek, Pavel Jungwirth (Academy of the Sciences, Prague)
Support by North-German Supercomputing Alliance (HLRN)
Ion pairing is an association of oppositely charged ions without formation of a covalent bond, where the mutual Coulomb attraction is partly shielded by the solvent. Typical arrangements range from contact ion pairs over solvent-shared ion pairs to solvent separated ion pairs. Using DFT molecular dynamics simulations we are able not only to determine locally H-bonded structural motifs but also to calculate free energy profiles by evaluating the ion-ion potential of mean force. These approaches do not only allow to identify deficiencies in conventional calculations of potentials of mean force but also to extract electronic information, such as the amount of charge transferred between ions and solvent and its dependence on the ion-ion distance.
These techniques have been applied to the challenging cases of high charge density ions, see our publication on aqueous solutions of the LiF ion pair . In a related study, we also investigated the binding of cations to peptide bonds modeling a protein backbone  as well as the formation and stability of salt bridges where protons may be transferred between acidic and basic side chains of a peptide. In our article on the Lys-Glu dipeptide in water we have focussed on the preference of different charge states as a function of the number of solvent molecules . In other work, the ion hydration and pairing in aqueous calcium chloride and formate/acetate solutions is investigated, where the DFT-MD simulations allow us to develop an accurate calcium force field which accounts in a mean-field way for electronic polarization effects via charge rescaling .
Solvating electrons in water
Burkhard Schmidt with Tomaso Frigato
Cooperation with Ondrej Maršálek, Frank Uhlig, Pavel Jungwirth (Academy of the Sciences, Prague)
Support by FU Berlin through CSS grant
Excess electrons in water are of fundamental importance as an intermediate reactive species in many areas of physics, chemistry and biology. Since its discovery in 1962, they have attracted a lot of attention, mainly due to their hugely delocalized structure, which is due to their highly quantal nature. However, the structural, energetic and spectroscopic aspects of negatively charged, finite size water aggregates are still under debate, e.g. the solvation of electrons at the surface or in the interior of clusters. Also the dynamics of the formation of cavities to accommodate an electron in water is still largely unexplored.
In our work, we have employed mixed quantum/classical DFT molecular dynamics simulation methods to address these questions, see our work on the structure of medium-sized water cluster anions where the computed electron binding affinities allow for a comparison with electron detachment experiments . In addition, we have also investigated the process of solvation of an initially delocalized electron as a function of the cluster temperature . In other work, the recombination reaction of electrons co-solvated with a proton has been characterized for the first time by means of ab initio molecular dynamics simulations .
Biomolecular conformations in aqueous solution
Burkhard Schmidt with Hui Zhu and Jens Antony
Cooperation with Susanna Röblitz, Marcus Weber (Zuse-Institut Berlin)
Support by DFG research center MATHEON (project A19)
Biomolecular conformations are - to a large extent - determined by the presence of the surrounding water. We strive at systematically investigating these effects of studying model peptides clusters with an increasing number of solvent particles, where we build on the concept of metastable sets of conformational space which often comprise many (almost isoenergetic) minima of the potential energy surface. On the longest time scales, the metastable "effective dynamics" is dominated by transitions between these conformations, while on shorter time scales, the dynamical behaviour is governed by flexibility within these conformations.
Reduced stochastic models can be constructed by a combination of Hidden Markov Model and Stochastic Differential Equations [E]. Alternatively, the molecular conformational space can be decomposed by spectral clustering techniques based on adaptive algorithms, see our conformation analysis of a tripeptide molecule , [F]. Different conformations of a peptide often exhibit small but characteristic changes of vibrational frequencies and intensities. In our work on a model tripeptide we establish a relation between conformational structures and corresponding mid-infrared spectra . In other work, we investigate non-adiabatic transitions between coupled amide I vibrational states of a model dipeptide triggered by conformational transitions .