BRIDGE

The theoretical understanding of solvation properties of molecules / macromolecules / interfaces in the domains of biology, colloidal physics, electrochemistry, etc., requires an explicit molecular solvent level of description with atom-atom classical force-fields. Beside the too much time-consuming numerical simulations, a powerful liquid physics theory has appeared recently, based on the 3D molecular DFT, which enables to calculate very quickly spatial+angular solvent density profiles as well as solvation free energy for any solute. Up to now, this MDFT approach is solved within the HNC approximation which neglects the so-called "bridge" correlation functions. It happens that for highly polar solvents presenting strong hydrogen bonding like water,HNC leads to quantitative disagreements. The BRIDGE project consists then in developing the theory beyond this standard approximation by constructing various bridge functionals or functions. The very wide experience of the team in statistical physics of liquids will make it possible to extend simple spherical liquid approaches to molecular solvents and solutes governed by highly anisotropic interactions and correlations. 4 routes will be explored: 1) We will first ignore the angular dependence and borrow the well-known hard-sphere functionals or construct simple polynomial functionals in a weighted spatial density. 2) We will solve the MDFT/HNC theory around a dimer made of the real solute and an additive solvent molecule at fixed relative position/orientation. That corresponds to use the HNC approximation higher in the hierarchy of the integral equations (for the dimer), so safer for the pair correlation. The solute-solvent bridge function will be extracted from the improved potential of mean force. 3) The exact functional can formally be written as an expansion involving a higher and higher number N of solvent molecules coupled via N-body direct correlation functions, the HNC approximation stopping at N=2. The present fundamental and ambitious route will calculate the next term, the 3-body bridge functional by extending what has been done in the literature for spheres. The bulk function c(3)(1,2,3) will be approximated by t(12)t(13)t(23) with the pair function t derived from a thermodynamical self-consistent relation. 4) This last approach will investigate an inverse route: the exact spatial/angular profiles will be provided by very precise simulation data and the statistical physics problem (Ornstein-Zernike + integral equations) will be inverted in order to produce exact solute-solvent bridge functions. Then, systematic behaviors will be drawn in order to reach the bridge functional. This ANR project will develop efficient theories, algorithms and numerical codes and will routinely solve MDFT and simulation for a great number of solutes. That requires important computing resources. Once optimized bridge functionals will be validated, MDFT will stand by itself as an efficient and competitive predictive tool, independent from simulations. It will be available to the community in a routinely manner in order to better understand real systems. Such applications will be investigated at the end of the project. The funding mainly corresponds to 3 years of postdoc and computers/workstations.