Water is the basic ingredient of our life. Despite its simple chemical formula, water exhib-its complexities that spread on a wide and disparate range of spatio-temporal scales: from the basic quantum nature of electrons and hydrogen bonds to the hydrodynamic-induced unfolding of proteins and stabilisation of membranes. Because of this large spectrum of scales, the adoption of a multi-scale approach when studying biological-relevant problems is particularly suitable because [1]. Different methodologies are under development. Coarse-grained potentials allow for large scale simulations for systems such as mem-branes [2]. However, consistency with experiments is a challenge [3], although encouraging approaches including hydrodynamics interactions [4], or shear flow [5], have been recently proposed. The following aspects will be covered:
1-Many body effects in water [6]: Most recent developments of ab initio functionals, elec-tronically coarse-grained methods, neural networks approach and force fields. Open ques-tion: How to model the cooperativity of water? Which strategies shall we adopt for the pa-rameterization of polarizable force fields? Is it realistic to think of a ‘universal model’ for water able to accurately reproduce its properties from gas to solid?
2- Toward multiscale modeling for bio-systems [7,8]: Biologically relevant problems in-volve a variety of length- and time-scale, from nano- to meso-scales. How can we perform the necessary coarse-graining without losing the properties that make water fundamental for the biological processes? Shall we develop a new model for each problem, or can we con-verge toward a rational and common strategy?
3- Phase diagram, extreme conditions and large-scale simulations [9]: Understanding of the different regions of the phase diagram, including a comparison of potential models and their reliability. How well is described the huge variety of phases of water? Are the ther-mophysical properties of water at extreme conditions important for biological systems?
4- Aqueous solutions and interfaces [10,11]: Influence of ions and interfaces on the hydro-gen bond network. How can we explain specific ion effects? Is it necessary to use polariza-ble force fields? How can we improve effective interaction potentials? Are there general ef-fects associated to interfaces or each interface is a new challenge?
5- Large bio-systems simulations [12,13]: Coarse graining at molecular level and extrapo-lation toward the continuum limit. How can we perform molecular coarse graining over wa-ter degrees of freedom keeping consistency with experiments? Are phenomenological, non-transferable models, inspired to colloidal science, the only possible solution? How does the hydrodynamics affect the validity of molecular models of water at biological conditions? Michael Levitt in his Nobel Lectures in 2013 pointed out that 'huge systems' are what we need to consider next: Are we there yet?