User Project Details

2DSFG

Computing the SFG and 2D-SFG spectra of salty water interfaces by DFT-based Molecular dynamics simulations

Chemistry

Universität Mainz

Institut für Physik

The aim of this continuation proposal is to provide a detailed justification of the computational resources, which we ask for the continuation of our project “Computing the SFG and 2D-SFG spectra of salty water interfaces by DFT-based Molecular dynamics simulations”, which was approved for the period 15.06.2017-15.06.2018, and which we would like to extend for the period 15.06.2018-15.06.2019. The main theme of our previous funding period has been the understanding of structure and dynamics of interfaces at the atomistic/molecular details. In particular we have been focusing on the development of a new simulation approach capable of providing a microscopic interpretation of vibrational energy relaxation as it occurs in vibrational pump- probe experiments. The idea is to produce vibrational excitations, localized in frequency and in space, and to follow their time evolution by non-equilibrium simulations. A distinctive feature of our approach is the use of ab initio molecular dynamics simulations, which include the full electronic structure and therefore naturally take into account mode anharmonicity and coupling between vibrational modes. In order to monitor the vibrational energy redistribution we have developed new descriptors, which are based on the vibrational density of states projected on the local normal modes. Our approach has been tested on bulk water. There we have followed the time evolution of both the stretching and the bending excitation obtaining relaxation time scales in very good agreement with those experimentally measured. Moreover, in collaboration with the molecular spectroscopy group of EHG Backus at the MPIP in Mainz we have also investigated the fluorite/water interfaces at different pH. Interesting differences have emerged when comparing low and high pH. As goal for the next funding period we aim to understand the molecular behavior of more complex interfaces, where in addition to the surfaces groups/charges an important role is played by the ions coming from the electrolyte solution, as well the role of a small adsorbate. The fundamental questions we aim to address are i) how the microscopic/ atomistic structure of the interfacial layer is modified in the presence of ions; ii) what drives some specific ions to accumulate at interfaces (Hofmeister series at the solid/liquid interface); iii) how the chemical properties of small adsorbates change as they adsorb at the solid/liquid interface. We will concentrate on two specific interfaces, namely the silica/water and the fluorite/ water interface which we have extensively investigated in different pH conditions and for which experimental data will be available from our collaborators at MPIP. As for the previous funding period, all the calculations will be performed with AIMD simulations (e.g. based on Density Functional Theory (DFT)), where the forces are calculated from the electronic structure. We believe that such an approach is the most suitable for an accurate description of heterogeneous condensed phase systems, such as interfaces, as it does not require an a priori parametrization. On the other hand, due to their huge computational cost, AIMD simulations calls for high performance computing resources, such as those which HRLS is able to provide.