The IPOQ 2025 workshop aims to advance on a series of closely interconnected open problems in molecular modelling and simulation of vapour-liquid and liquid-liquid interfaces:

Thermodynamic properties: Systematization and ontologization. Research data management and interoperability between software components today require metadata standardization and therefore a systematization of thermodynamic properties. Approaches to be discussed include the EMMO [1] and the Massieu-Planck formalism [2]. They have so far mainly addressed bulk properties – what is the best way forward on interfacial properties?

Methods for curved interfaces, finite-size effects, and nucleation. The surface tension of nanodroplets/-bubbles is influenced by curvature and small size. It is a challenge to agree on simulation methods for nanodispersed phases, since methods disagree on the surface tension [3]. This controversy relates to defining the property computed under the name «surface tension». We aim to advance at least on the terminology to be used, if not on the methods, and focus on test-transformation methods as well as expressions and methods that are deduced from infinitesimal transformations [4]. What are they in fact computing, and is it the same «surface tension» as e.g. the one obtained from the excess grand potential?

Simultaneous molecular model accuracy for both interfacial and bulk properties. While molecular models with adjustable parameters can achieve near-perfect agreement on one or two thermodynamic properties simultaneously, there is a limitation when an increasing number of optimization objectives is to be considered during parameterization. But between research groups and their approaches, there are stark disagreements on how successfully interfacial and bulk properties can be reconciled with each other. Here we aim to understand each other better and figure out why some modelling approaches seem to be extremely adjustable and even predictive [5], while other, rather similar modelling frameworks appear much more limited [6, 7].

Surface tension, virial, and pressure tensor in simulations with rigid multi-site models. We discuss correction schemes to be applied when computing the surface tension using multi-site models without internal degrees of freedom [8]. We hope to advance on a controversial issue: Is there such a thing as the «ideal gas contribution» to the surface tension, coming from the kinetic energy’s contribution to the pressure tensor [9, 10]?

Multiscale and mesoscopic modelling of interfaces: How consistent can it become? Intermolecular pair potentials for MC/MD simulation are increasingly treated as part of comprehensive approaches including molecular equations of state, density gradient theory, phase field, LB, or DPD simulation [5]. This is successful, but poses the risk of inconsistencies [1]. What mathematical and ontological tools should we use to deal with this?

Interfacial micromechanics: Localized quantities, intrinsic interfaces, and capillary waves. The outcome from previous sessions is complemented through a dedicated perspective on micromechanics, i.e., mechanically inspired properties at the molecular level. To what extent are we speaking the same language as in thermodynamics and statistical mechanics when we apply micromechanics, or where that is not the case, how well can we translate between paradigms? We focus on two established micromechanical notions: The intrinsic surface [10, 11], and the property interfacial enrichment [12].

[1] H. A. Preisig, T. F. Hagelien, J. Friis, P. Klein, N. Konchakova, «Ontologies in computational engineering», in Proc. ECCOMAS 2020, Scipedia, p. 262, doi:10.23967/wccm-eccomas.2020.262, 2021.

[2] R. Lustig, «Statistical analogues for fundamental equation of state derivatives», Mol. Phys. 110(24): 3041–3052, doi:10.1080/00268976.2012.695032, 2012.

[3] M. Horsch, H. Hasse, A. K. Shchekin, A. Agarwal, S. Eckelsbach, J. Vrabec, E. A. Müller, G. Jackson, «Excess equimolar radius of liquid drops», Phys. Rev. E 85: 031605, doi:10.1103/PhysRevE.85.031605, 2012.

[4] G. V. Lau, I. J. Ford, P. A. Hunt, E. A. Müller, G. Jackson, «Surface thermodynamics of planar, cylindrical and spherical vapour-liquid interfaces», J. Chem. Phys. 142: 114701, doi:10.1063/1.4913371, 2015.

[5] J. M. Garrido, M. M. Piñeiro, F. J. Blas, E. A. Müller, «Interfacial tensions of industrial fluids from a molecular-based square gradient theory», AIChE J. 62(5): 1781 – 1794, doi:10.1002/aic.15190, 2016.

[6] S. Werth, K. Stöbener, P. Klein, K.-H. Küfer, M. Horsch, H. Hasse, «Molecular modelling and simulation of the surface tension of real quadrupolar fluids», Chem. Eng. Sci. 121: 110–117, doi:10.1016/j.ces.2014.08.035, 2015.

[7] S. Werth, M. Horsch, H. Hasse, «Molecular simulation of the surface tension of 33 multi-site models for real fluids», J. Mol. Liq. 235: 126–134, doi:10.1016/j.molliq.2016.12.062, 2017; J. Mol. Liq. 286: 110877, doi:10.1016/j.molliq.2019.110877, 2019.

[8] S. Werth, G. Rutkai, J. Vrabec, M. Horsch, H. Hasse, «Long-range correction for multi-site Lennard-Jones models and planar interfaces», Mol. Phys. 112(17): 2227–2234, doi:10.1080/00268976.2013.861086, 2014.

[9] M. Sega, B. Fábian, P. Jedlovszky, «Nonzero ideal gas contribution to the surface tension of water», J. Phys. Chem. Lett. 8(12): 2608–2612, doi:10.1021/acs.jpclett.7b01024, 2017.

[10] M. Lbadaoui-Darvas, G. Garberoglio, K. S. Karadima, M. N. D. S. Cordeiro, A. Nenes, S. Takahama, «Molecular simulations of interfacial systems: Challenges, applications and future perspectives», Mol. Sim., doi:10.1080/08927022.2021.1980215, 2023.

[11] J. Hernández Muñoz, P. Tarazona, E. Chacón, «Layering and capillary waves in the structure factor of liquid surfaces», J. Chem. Phys. 157: 154703, doi:10.1063/5.0118252, 2022.

[12] S. Stephan, H. Hasse, «Enrichment at vapour-liquid interfaces of mixtures: Establishing a link between nanoscopic and macroscopic properties», Int. Rev. Phys. Chem. 39(3): 319–349, doi:10.1080/0144235X.2020.1777705, 2020.