Identifying interfacial molecules in nonplanar interfaces: the generalized ITIM algorithm

TL;DR

This paper introduces a generalized ITIM algorithm capable of identifying interfacial molecules on arbitrarily shaped interfaces, improving analysis of complex systems like micelles and soot structures.

Contribution

It extends the ITIM algorithm by integrating alpha-shapes concepts, enabling reference-frame independent identification of interfacial molecules in nonplanar geometries.

Findings

Applied to water around micelles and soot structures.

Enabled calculation of intrinsic density profiles in complex geometries.

Improved understanding of interfacial molecular arrangements.

Abstract

We present a generalized version of the ITIM algorithm for the identification of interfacial molecules, which is able to treat arbitrarily shaped interfaces. The algorithm exploits the similarities between the concept of probe sphere used in ITIM and the circumsphere criterion used in the alpha-shapes approach, and can be regarded either as a reference-frame independent version of the former, or as an extended version of the latter that includes the atomic excluded volume. The new algorithm is applied to compute the intrinsic orientational order parameters of water around a DPC and a cholic acid micelle in aqueous environment, and to the identification of solvent-reachable sites in four model structures for soot. The additional algorithm introduced for the calculation of intrinsic density profiles in arbitrary geometries proved to be extremely useful also for planar interfaces, as it allows to solve the paradox of smeared intrinsic profiles far from the interface