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Electronic coarse graining enhances the predictive power of molecular simulation allowing challenges in water physics to be addressed

One key factor that limits the predictive power of molecular dynamics simulations is the accuracy and transferability of the input force field. Force fields are challenged by heterogeneous environments, where electronic responses give rise to biologically important forces such as many-body polarisation and dispersion. The importance of polarisation was recognised early-on and described by Cochran in 1959 [Philosophical Magazine 4 (1959) 1082-1086]. However, dispersion forces are still treated at the two-body level and in the dipole limit, although the importance of three-body terms in the condensed phase was demonstrated by Barker in the 1980s [Phys. Rev. Lett. 57 (1986) 230-233]. A way of treating both polarisation and dispersion on an equal basis is to coarse grain the electrons a molecular moiety to a single quantum harmonic oscillator, as suggested as early as the 1960s by Hirschfelder, Curtiss and Bird [The Molecular Theory of Gases and Liquids (1954)]. This treatment, when solved in the strong coupling limit, gives all orders of long-range forces. In the last decade, the tools necessary to exploit this strong coupling limit have been developed, culminating in a transferable model of water with excellent predictive power across the phase diagram. This transferability arises since the environment identifies the form of long range interactions, rather than the expressions selected by the modeller. Here, we discuss the role of electronic coarse-graining in predictive multiscale materials modelling and describe the first implementation of the method in a general purpose molecular dynamics software, QDO_MD.