  Information retrieval (IR) plays a crucial role in locating relevant
resources from vast amounts of data, and its applications have evolved from
traditional knowledge bases to modern retrieval models (RMs). The emergence of
large language models (LLMs) has further revolutionized the IR field by
enabling users to interact with search systems in natural languages. In this
paper, we explore the advantages and disadvantages of LLMs and RMs,
highlighting their respective strengths in understanding user-issued queries
and retrieving up-to-date information. To leverage the benefits of both
paradigms while circumventing their limitations, we propose InteR, a novel
framework that facilitates information refinement through synergy between RMs
and LLMs. InteR allows RMs to expand knowledge in queries using LLM-generated
knowledge collections and enables LLMs to enhance prompt formulation using
retrieved documents. This iterative refinement process augments the inputs of
RMs and LLMs, leading to more accurate retrieval. Experiments on large-scale
retrieval benchmarks involving web search and low-resource retrieval tasks
demonstrate that InteR achieves overall superior zero-shot retrieval
performance compared to state-of-the-art methods, even those using relevance
judgment. Source code is available at https://github.com/Cyril-JZ/InteR
