Variational methods for contracting projected entangled-pair states

TL;DR

This paper introduces a variational approach to contracting PEPS, enabling comparison of existing algorithms and proposing a new scheme for computing correlation functions, improving accuracy and consistency.

Contribution

It identifies a subclass of PEPS allowing an algorithm-independent variational formulation, and develops a new contraction method for N-point correlations.

Findings

The variational formulation enables assessment of contraction algorithms.

The new scheme extends to general N-point correlation functions.

Comparison shows differences in accuracy among existing methods.

Abstract

The norms or expectation values of infinite projected entangled-pair states (PEPS) cannot be computed exactly, and approximation algorithms have to be applied. In the last years, many efficient algorithms have been devised -- the corner transfer matrix renormalization group (CTMRG) and variational uniform matrix product state (VUMPS) algorithm are the most common -- but it remains unclear whether they always lead to the same results. In this paper, we identify a subclass of PEPS for which we can reformulate the contraction as a variational problem that is algorithm independent. We use this variational feature to assess and compare the accuracy of CTMRG and VUMPS contractions. Moreover, we devise a new variational contraction scheme, which we can extend to compute general N-point correlation functions.