# Equilibrium properties of the lattice system with SALR interaction potential on a square lattice: quasi-chemical approximation versus Monte Carlo simulation

## TL;DR

This paper investigates the equilibrium properties of a lattice system with SALR interactions on a square lattice, comparing a quasi-chemical approximation with Monte Carlo simulations to understand phase behavior and order parameters.

## Contribution

It introduces a quasi-chemical approximation for the SALR lattice system and compares its results with Monte Carlo simulations, demonstrating its semi-quantitative accuracy.

## Key findings

- QChA provides reasonable semi-quantitative results.
- Phase diagram and critical order parameter are estimated.
- Monte Carlo simulations validate the approximation.

## Abstract

The lattice system with competing interactions that models biological objects (colloids, ensembles of protein molecules, etc.) is considered. This system is the lattice fluid on a square lattice with attractive interaction between nearest neighbours and repulsive interaction between next-next-nearest neighbours. The geometric order parameter is introduced for describing the ordered phases in this system. The critical value of the order parameter is estimated and the phase diagram of the system is constructed. The simple quasi-chemical approximation (QChA) is proposed for the system under consideration. The data of Monte Carlo simulation of equilibrium properties of the model are compared with the results of QChA. It is shown that QChA provides reasonable semiquantitative results for the systems studied and can be used as the basis for next order approximations.