Octopus, a computational framework for exploring light-driven phenomena and quantum dynamics in extended and finite systems

Abstract: Over the last years extraordinary advances in experimental and theoretical tools have allowed us to monitor and control matter at short time and atomic scales with a high-degree of precision. An appealing and challenging route towards engineering materials with tailored properties is to find ways to design or selectively manipulate materials, especially at the quantum level. To this end, having a state-of-the-art ab initio computer simulation tool that enables a reliable and accurate simulation of light-induced changes in the physical and chemical properties of complex systems is of utmost importance. The first principles real-space-based Octopus project was born with that idea in mind, providing an unique framework allowing to describe non-equilibrium phenomena in molecular complexes, low dimensional materials, and extended systems by accounting for electronic, ionic, and photon quantum mechanical effects within a generalized time-dependent density functional theory framework. The present article aims to present the new features that have been implemented over the last few years, including technical developments related to performance and massive parallelism. We also describe the major theoretical developments to address ultrafast light-driven processes, like the new theoretical framework of quantum electrodynamics density-functional formalism (QEDFT) for the description of novel light-matter hybrid states. Those advances, and other being released soon as part of the Octopus package, will enable the scientific community to simulate and characterize spatial and time-resolved spectroscopies, ultrafast phenomena in molecules and materials, and new emergent states of matter (QED-materials).

• The paper introduces an advanced QEDFT method that explicitly simulates photon interactions, enabling accurate modeling of light-matter hybrid states.
• It implements coupled Maxwell-Kohn-Sham equations for self-consistent nano-optical simulations, enhancing the study of plasmonic excitations.
• The work demonstrates significant code improvements, scalability, and rigorous benchmarking, ensuring efficient and reliable ab initio simulations.

Overview of "Octopus, a computational framework for exploring light-driven phenomena and quantum dynamics in extended and finite systems"

The paper introduces the Octopus software framework, a versatile tool designed to simulate and study light-induced quantum phenomena in both extended and finite systems using ab initio approaches. The framework focuses particularly on phenomena where quantum mechanical treatment of electrons, ions, and photons is essential, utilizing a real-space time-dependent density functional theory (TDDFT) approach. This paper details the new computational features implemented in Octopus, providing insights into both the theoretical advancements and technical improvements that have been made.

Key Contributions:

1. Quantum Electrodynamics TDDFT (QEDFT): The paper highlights the integration of QEDFT, a method within Octopus that allows for the simulation of systems with strong light-matter interactions, such as those involving cavity quantum electrodynamics. This marks an advance beyond conventional TDDFT by incorporating photon interactions explicitly, which is crucial for accurately simulating new light-matter hybrid states.
2. Multi-scale Couplings and Electromagnetic Fields: Octopus includes a novel implementation of coupled Maxwell-Kohn-Sham equations, allowing for self-consistent light-matter interaction simulations. This feature specifically targets nano-optical applications, providing enhanced capability to explore phenomena like plasmonic excitations in nanostructures.
3. Advanced Simulation Techniques: Several technical enhancements have been made to the code, such as the use of new numerical propagators and algorithms to handle electron dynamics more efficiently. This includes conjugate gradient methods, improved real-space preconditioners, and advanced spectral analysis tools.
4. Resource Management and Scalability: The paper describes significant efforts in improving Octopus's parallel performance, making the framework suitable for high-performance computing environments. Notably, the implementation of GPU support and other optimization techniques ensures Octopus can efficiently handle large-scale simulations across distributed computing platforms.
5. Benchmarking and Validation: The researchers have performed extensive testing, such as with the Delta-factor test for periodic systems, to validate the accuracy and reliability of the framework. This ensures that Octopus can provide robust results across a variety of system types and conditions.

Implications and Future Directions:

The Octopus framework sets a foundation for future research in several emerging fields within physics and materials science. Given its capability to simulate complex light-induced processes, Octopus is positioned to drive forward applications in nonequilibrium quantum dynamics, photovoltaics, and the design of new functional materials with tailored optical properties. The continued development of Octopus will likely focus on expanding its methodological reach and further integrating components that address correlated electron systems, facilitate open quantum system studies, and exploit artificial intelligence in simulation processes.

Overall, Octopus represents a significant computational tool that bridges theoretical advancements with practical applications, enabling the exploration of quantum phenomena with unprecedented detail and accuracy. Its development highlights ongoing collaborative efforts in computational physics to advance understanding of complex quantum systems and how they can be manipulated through external fields, setting the stage for novel technological applications.