Getting the intermolecular forces correct: introducing the ASTA strategy for a water model

Having a force field for water providing good bulk properties is paramount for modern studies of most biological systems. Some of the most common three-site force fields are TIP3, SPC/ϵ or OPC3, providing a decent range of bulk properties. That does not mean though, that they have realistic inter- atomic forces. These force fields have been parameterized with a top-down approach, meaning, by fitting the force field parameters to the experimental bulk properties. This approach grows in inaccuracy the more different the studied system is from the originally parameterized force field. We test a bottom-up approach, in which the force field is parameterized by optimizing the non-bonded inter-atomic forces within a general framework and thus, not dependent on the nature of the system. Our philosophy is that correct inter-atomic forces lead to correct geometrical and dynamical properties. The first system we try to optimize with the accurately system tailored atomic (ASTA) algorithm is water, but we aim to eventually probe other systems in the future as well. We applied our ASTA strategy to find a good set of parameters providing accurate bulk properties for the simple three-site force field forms, and also, for more physical polarizable force fields, such as AMOEBA. Even though our bottom-up approach did not provide satisfactory results for the simple three-site force fields (with fixed charges), for the case of the AMOEBA force field it led to parameters giving very good intra- and inter-molecular forces, as compared to accurate quantum chemically calculated reference forces. At the same time, important bulk properties, such as the density and diffusion, were accurately reproduced with respect to the experimental values.