# Non-Hermitian coupled cluster method for non-stationary systems and its interaction-picture reinterpretation

## TL;DR

This paper introduces a non-Hermitian interaction-picture framework for many-body quantum physics, extending coupled cluster methods to non-stationary systems and revealing parallels with time-dependent CCM techniques.

## Contribution

It develops a non-Hermitian interaction-picture for non-stationary quantum systems and links it with time-dependent coupled cluster methods, offering new tools for many-body quantum analysis.

## Key findings

- Formulation of a non-Hermitian interaction-picture (NHIP) for dynamics.
- Identification of parallels between NHIP and time-dependent CCM.
- Potential for improved modeling of non-stationary quantum systems.

## Abstract

The interaction picture in a non-Hermitian realization is discussed in detail and considered for its practical use in many-body quantum physics. The resulting non-Hermitian interaction-picture (NHIP) description of dynamics, in which both the wave functions and operators belonging to physical observables cease to remain constant in time, is a non-Hermitian generalization of the traditional Dirac picture of standard quantum mechanics, which itself is widely used in quantum field theory calculations. Particular attention is paid here to the variational (or, better, bivariational) and dynamical (i.e., non-stationary) aspects that are characteristic of the coupled cluster method (CCM) techniques that nowadays form one of the most versatile and most accurate of all available formulations of quantum many-body theory. In so doing we expose and exploit multiple parallels between the NHIP and the CCM in its time-dependent versions.