Zero temperature phase diagram of the square-shoulder system

TL;DR

This paper explores the zero-temperature phase diagram of particles with a square-shoulder potential, revealing a sequence of self-organized structures as pressure increases, from clusters to high-symmetry lattices.

Contribution

It introduces a systematic optimization approach using genetic algorithms to map the phase diagram of square-shoulder particles at zero temperature.

Findings

Sequence of structures from clusters to lattices with increasing pressure

Identification of low-symmetry and high-symmetry phases

Structures characterized by minimal corona overlap and maximal density

Abstract

Particles that interact via a square-shoulder potential, consisting of an impenetrable hard core with an adjacent, repulsive, step-like corona, are able to self-organize in a surprisingly rich variety of rather unconventional ordered structures. Using optimization strategies that are based on ideas of genetic algorithms we encounter, as we systematically increase the pressure, the following archetypes of aggregates: low-symmetry cluster and columnar phases, followed by lamellar particle arrangements, until at high pressure values compact, high-symmetry lattices emerge. These structures are characterized in the NPT ensemble as configurations of minimum Gibbs free energy. Based on simple considerations, i.e., basically minimizing the number of overlapping coronae while maximizing at the same time the density, the sequence of emerging structures can easily be understood.