The formation of ice has an impact across global phenomena such as climate change [1] (ice is a key component of the clouds in our atmosphere) as well as on the microscopic details of water freezing within our own cells [2] (which is of the greatest relevance for cryopreservation). Invariably, though, ice forms heterogeneously, thanks to the presence of impurities boosting the otherwise too low ice nucleation rate of pure water [1,2].

In the last few years we have achieved a good understanding of which substances can promote heterogeneous ice nucleation (HIN): however, we still lack the microscopic insight that would allow us to understand (and predict!) the ice nucleating ability of a given substrate. This is because experiments still struggle to characterize crystal nucleation, which happens on exceedingly small/short length/time scales (ns/nm) [3]. Conversely, molecular simulations, which could indeed provide invaluable insight, are hampered by the fact that [3]:

• Accurate interatomic potentials/force fields describing water-water and water-substrate interactions at the same time are needed in order to perform reliable simulations of HIN, typically via classical molecular dynamics. Building such force fields has proven to be an incredibly challenging task.
• Nucleation is a rare event, as seconds, or days or even weeks are typically needed for a crystalline nucleus to reach its critical size and proceed toward crystallization. Thus, enhanced sampling techniques are almost always needed to tackle the time scale problem via molecular.
• We are very far away from being able to compare the results of simulations and experiments. For instance, one of the very few quantities that could in principle link the two is the nucleation rate, but a quantitative agreement still eludes us.

The aim of the workshop is to address these issues by devising practical strategies to further the scope, the reliability and the impact of molecular simulations of HIN, in order to bring the latter a step closer to experiments.

We build upon the CECAM Workshop "From Atoms to Clouds", held in 2014 in Zurich. This was a success, as it prompted a number of scientific collaborations - one of which led to a paper published in Science [4]. However, within three years from that Workshop we have witnessed amazing advancements in the field of HIN: new enhanced sampling techniques have emerged [5], experiments can now image ice nucleation sites [4], and the first attempts to characterize HIN on biological matter have been reported [6]. Hence the title of this Workshop, " Heterogeneous Ice Nucleation: The Ultimate Challenge for Molecular Modelling?": in contrast with the 2014 Workshop, we will address the practical challenges that simulations will have to overcome in the years to come, and in doing so we will by no means restrict the discussion to atmospheric science. In fact, we have selected as Invited Speakers a blend of the best people with expertise in modelling and experiments of HIN alike: we are thus confident this Workshop will be pivotal in order to take molecular simulations of HIN to the next level. 

What's on

• Taking stock:
  - What do we know about supercooled water that could help us understand the subtleties of Heterogeneous Ice Nucleation (HIN)?
  - Water on mineral and biological surfaces: how can simulations deal with this level of complexity? Are coarse grained models the solution?
  - What is it that is preventing the computational community to investigate the formation of ice on biological matter? 
  -- Opportunities:
  To put together a critical assessment of the computational work that has been done in the field of HIN, in order to build upon this expertise and identifying the most pressing questions to be answered in the near and far future alike.

• Force fields:
  - Simulations of HIN are delicate: the impact of the computational setup, the flexibility of the substrate...
  - The future of Force Fields: Whether we need better water/interfaces models in the context of heterogeneous ice nucleation - and how could we proceed to construct them.
  -- Opportunities:
  To develop improved water/surface models, with the guidance of experiments and taking advantage of novel approaches such as machine learning-based algorithms.

• Simulation Methods & Enhanced Sampling (ES):
  - Free energy based ES methods versus path sampling ES methods: pros and cons. 
  - Seeded molecular dynamics: hugely successful for homogenous nucleation - can we extend this framework to HIN?
  - Software Medley: How to further software development throughout e.g. collaborative coding.
  -- Opportunities:
  To devise and implement novel enhanced sampling techniques capable of making molecular simulations HIN more accurate and computationally affordable.

• Clathrate Hydrates:
  - There has been incredibly scarce study on heterogeneous hydrate nucleation: the role of surface in enhancing is much more elusive and controversial, and so it is the role of inhibitors. 
  -- Opportunities:
  To engage the computational community toward impact of practical relevance for the oil and gas industries, as the formation of hydrates is usually costly (pipelines block) and dangerous.

• Bridging the Gap:
  - Getting Closer to Experiments (Open Discussion D): How to maximize the impact of simulations with respect to experiments and applications
  - What do atmospheric scientists and cryobiologists need from simulations of HIN?
  - What are the quantitative and qualitative results that simulations can hope to compare with experimental results?
  -- Opportunities:
  -To harness experimental insight (e.g. surface topography) in order to improve simulations.
  -To focus experimental work on specific (more accessible computationally) systems

• Future challenges:
  - From flat, pristine inorganic crystalline (and biological!) surfaces to the actual topographies encountered experimentally (defects, different ice nucleating sites): how do we bridge the gap?
  - What are the cutting-edge experimental techniques which can hope to achieve the spatial and temporal resolutions characteristic of HIN?
  - Pinpointing the Challenges: The most pressing challenges for simulations of HIN to be tackled.
  -- Opportunities:
  Getting a group of relevant people together could lead to joint European projects/proposals