# Implementation of gauge-invariant time-dependent configuration interaction singles method for three-dimensional atoms

## TL;DR

This paper introduces a gauge-invariant, computationally efficient implementation of the time-dependent configuration interaction singles method for three-dimensional atoms, enabling improved simulations of high-field phenomena like high-harmonic generation.

## Contribution

It presents a novel gauge-invariant TDCIS implementation using channel orbitals, enhancing computational efficiency and physical consistency in atomic high-field simulations.

## Key findings

- Confirmed gauge invariance in simulations
- Demonstrated effectiveness of velocity gauge treatment
- Applied to high-harmonic generation in helium and neon

## Abstract

We present a numerical implementation of the gauge-invariant time-dependent configuration interaction singles (TDCIS) method [Appl. Sci. 8, 433 (2018)] for three-dimensional atoms. In our implementation, orbital-like quantity called channel orbital [Phys. Rev. A 74, 043420 (2006)] is propagated instead of configuration-interaction (CI) coefficients, which removes a computational bottleneck of explicitly calculating and storing numerous virtual orbitals. Furthermore, besides its physical consistency, the gauge-invariant formulation allows to take advantages of the velocity gauge treatment of the laser-electron interaction over the length gauge one in the simulation of high-field phenomena. We apply the present implementation to high-harmonic generation from helium and neon atoms, and numerically confirms the gauge invariance and demonstrates the effectiveness of the rotated velocity gauge treatment.