Numerical Methods for the QCD Overlap Operator IV: Hybrid Monte Carlo

TL;DR

This paper presents an adaptation of the Hybrid Monte Carlo algorithm for overlap fermions, utilizing rational approximation and eigenvalue treatment to reduce computational costs and improve simulation feasibility on medium-sized lattices.

Contribution

It introduces a new HMC algorithm for overlap fermions with improved force calculation and energy conservation, enabling more efficient lattice QCD simulations.

Findings

Energy violations are better than O(Δτ²)

Algorithm satisfies reversibility and area conservation

Tested successfully on small lattices

Abstract

The extreme computational costs of calculating the sign of the Wilson matrix within the overlap operator have so far prevented four dimensional dynamical overlap simulations on realistic lattice sizes, because the computational power required to invert the overlap operator, the time consuming part of the Hybrid Monte Carlo algorithm, is too high. In this series of papers we introduced the optimal approximation of the sign function and have been developing preconditioning and relaxation techniques which reduce the time needed for the inversion of the overlap operator by over a factor of four, bringing the simulation of dynamical overlap fermions on medium-size lattices within the range of Teraflop-computers. In this paper we adapt the HMC algorithm to overlap fermions. We approximate the matrix sign function using the Zolotarev rational approximation, treating the smallest eigenvalues of the Wilson operator exactly within the fermionic force. We then derive the fermionic force for the overlap operator, elaborating on the problem of Dirac delta-function terms from zero crossings of eigenvalues of the Wilson operator. The crossing scheme proposed shows energy violations which are better than O($\Delta\tau^2$) and thus are comparable with the violations of the standard leapfrog algorithm over the course of a trajectory. We explicitly prove that our algorithm satisfies reversibility and area conservation. Finally, we test our algorithm on small $4^4$, $6^4$, and $8^4$ lattices at large masses.