The focus of my research is bridging scales in
condensed matter and material science using computational approaches.
Below are described the main directions:
(a)Development of Computational Methodologies
(b)Applications
This line of research involves two distinct subjects:
Development of Computational Methodologies:
(i)The Adaptive Resolution Simulation Scheme (AdResS):,
I have developed the basis of this method with Matej Praprotnik and Kurt Kremer. The method consists of a computational scheme which allows to decide on demand the level of details of a
spatial region in a simulation run by changing molecular resolution, and thus the
number of degrees of freedom, on the fly while the molecule transits from a
high resolution region to a lower one (and vice versa). A theoretical framework has been constructed and employed to determine the
statistical properties in the region of varying resolution. Contrary to the standard MD schemes, this one is not based on the conservation of energy but yet the theoretical framework on which is based allows for a control of the thermodynamics and of the statistical equilibrium of the system (work of Simon Poblete).
While Matej Praprotnik and Kurt Kremer have extended this approach to the coupling with the continuum (with Rafael Delgado-Buscalioni) I have gone towards the quantum-classical adaptive by extending AdResS to the path integral description of the atoms as high resolution approach (work of Adolfo Poma).
(ii)Analytic approach and Monte Carlo sampling for electron correlations:
I have derived a generalization of the Levy
constrained-search method which then allows to design an internally consistent computational protocol
involving the Monte Carlo Sampling of electronic configurations in space (with Luca Ghiringhelli). Using
these procedure one can numerically obtain a local kinetic functional. Surprisingly, we found a functional form proportional to the Shannon entropy and I have later given an interpretation, via a scaling analysis, within the Density Functional Theory framework.
Interestingly, the explicit inclusion of the spin into the description of the system leads to a further term that also is proportional to the Shannon entropy. Our results suggest that indeed there may be a non trivial link between many-electron and Information theory as suggested by several authors who instead based their conclusions on empirical arguments. This research may, in a not far future, be of help for the design of electronic Kinetic Functionals for Orbital Free Density Functional (OFDFT) based codes.
Applications:
(ii)Solvation of ions in water:
With Christian Krekeler, we have studied the effects of mono and
divalent ions onto the structure of the solvation shells of water
around the ion. We have shown that this effect is confined
to the first solvation shell only regardless of the size and the charge of the
ion. This is in agreement with recent experiments and in contradiction
to the commonly accepted concept that the dominant interaction in the solvation
process is that of ion-water. We have also provide a justification of the
results found in terms of electronic polarization which led to the
counterintuitive conclusion that such a behaviour is due to the distortion of
the molecular orbitals caused by the interaction between non-hydrogen bonded
water molecules in the first solvation shell of the ion.
(iii)Multiscale modeling of Ionic Liquids:
This is a project funded by the DFG, within the priority program SP1191, in collaboration
with the group of Christian Holm in Stuttgart and that of Robert Berger at the FIAS in
Frankfurt/Darmstadt. The aim is to build a systematic methodology to cover the scales from
the high level quantum chemical approach, to the DFT up to the classical
MD. Such a methodology should be based on studying systems of increasing size
going from one level to the other and modeling the classical scale in a way that
some consistency in structures and potentials with the quantum methods is
found. So far in my group we have shown from CPMD calculation that the cation and anion dipoles are highly fluctuating but the electrostatic properties are very local. This suggest that standard non polarizable force fields may not be appropriate to study these systems.