Computation of microcanonical entropy at fixed magnetization without direct counting

TL;DR

This paper introduces a saddle-point based method to compute the microcanonical entropy at fixed magnetization, avoiding direct counting, and demonstrates its effectiveness on complex Ising models with various interactions.

Contribution

The paper presents a novel saddle-point optimization approach for calculating microcanonical entropy without direct enumeration, applicable to complex spin systems.

Findings

Method agrees with direct counting for benchmark models

Efficiently handles models with complex interactions

Outperforms direct counting in computational convenience

Abstract

We discuss a method to compute the microcanonical entropy at fixed magnetization without direct counting. Our approach is based on the evaluation of a saddle-point leading to an optimization problem. The method is applied to a benchmark Ising model with simultaneous presence of mean-field and nearest-neighbour interactions for which direct counting is indeed possible, thus allowing a comparison. Moreover, we apply the method to an Ising model with mean-field, nearest-neighbour and next-nearest-neighbour interactions, for which direct counting is not straightforward. For this model, we compare the solution obtained by our method with the one obtained from the formula for the entropy in terms of all correlation functions. This example shows that for general couplings our method is much more convenient than direct counting methods to compute the microcanonical entropy at fixed magnetization.