We propose a workshop on innovative approaches to the computational studies of droplets with multiple ions. To our knowledge, CECAM has never hosted a workshop in this research direction. The theoretical and computational study of these systems aims towards establishing universal principles and theories to explain and predict the properties of charged droplets in a wide range of contexts. The study of droplets with multiple ions would benefit from findings in several other fields, such as liquid-vapour interfaces, solvation of ions in clusters, and modelling of droplet fluctuations in nuclear physics. We therefore plan to involve participants with a wide range of backgrounds who might not otherwise interact.

In the experimental field of electrospray ionization the critical issue is control of the factors that determine the charge state of macromolecules. The fragmentation pattern is one of the key considerations at play here. However, fragmentation is coupled to a multitude of other processes that occur in a droplet, including charge-induced instabilities of the droplet due to the presence of macromolecular ions [14,15], chemical reactions within the droplet, and solvent evaporation [16,17]. A major obstacle in the understanding of the behaviour of macromolecules in droplets is that their charge state changes in the droplet environment [18]. This effect has been studied by stochastic modelling of the charging of proteins [18] and has not been taken into account in current molecular dynamics simulations of proteins drying out from droplets.

1. To what extent do quantum calculations enhance our understanding of the solvation of multiple ions in droplets?

2. What methodology is appropriate for the modelling of protonation reactions involving macromolecules such as peptides in a droplet? These reactions affect the charge state of the macromolecule. This question will benefit from the input of scientists in the fields of quantum modelling coupled to molecular mechanics.

3. How can the statistical sampling of reactive events be performed efficiently?

4. Simulations of large droplets are very time consuming because of the need to treat long-range electrostatic interactions in a finite system. The combination of short-range cut-offs and Ewald summation techniques used in periodic systems is not applicable. How can electrostatics be efficiently treated in cluster environments in order to make the simulations faster?

5. What is the effect of the droplet environment in the conformational changes of macromolecules?

6. The most ambitious question to answer is whether simulation methods can be used to predict the charge state of a macromolecule and provide a prescription for how to manipulate such a state.