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Ultrafast electron dynamics have made rapid progress in the last few years. With Jellyfish, we now introduce a program suite that enables to perform the entire workflow of an electron‐dynamics simulation. The modular program architecture offers a flexible combination of different propagators, Hamiltonians, basis sets, and more. Jellyfish can be operated by a graphical user interface, which makes it easy to get started for nonspecialist users and gives experienced users a clear overview of the entire functionality. The temporal evolution of a wave function can currently be executed in the time‐dependent configuration interaction method (TDCI) formalism, however, a plugin system facilitates the expansion to other methods and tools without requiring in‐depth knowledge of the program. Currently developed plugins allow to include results from conventional electronic structure calculations as well as the usage and extension of quantum‐compute algorithms for electron dynamics. We present the capabilities of Jellyfish on three examples to showcase the simulation and analysis of light‐driven correlated electron dynamics. The implemented visualization of various densities enables an efficient and detailed analysis for the long‐standing quest of the electron–hole pair formation. This article is categorized under:Theoretical and Physical Chemistry > SpectroscopySoftware > Simulation Methods [ABSTRACT FROM AUTHOR]

Polaritonic chemistry is an interdisciplinary emerging field that presents several challenges and opportunities in chemistry, physics, and engineering. A systematic review of polaritonic response theory is presented, following a chemical perspective based on molecular response theory. We provide the reader with a general strategy for developing response theory for ab initio cavity quantum electrodynamics (QED) methods and critically emphasize details that still need clarification and require cooperation between the physical and chemistry communities. We show that several well‐established results can be applied to strong coupling light‐matter systems, leading to novel perspectives on the computation of matter and photonic properties. The application of the Pauli–Fierz Hamiltonian to polaritons is discussed, focusing on the effects of describing operators in different mathematical representations. We thoroughly examine the most common approximations employed in ab initio QED, such as the dipole approximation. We introduce the polaritonic response equations for the recently developed ab initio QED Hartree–Fock and QED coupled cluster methods. The discussion focuses on the similarities and differences from standard quantum chemistry methods, providing practical equations for computing the polaritonic properties. This article is categorized under:Electronic Structure Theory > Ab Initio Electronic Structure MethodsTheoretical and Physical Chemistry > SpectroscopySoftware > Quantum Chemistry [ABSTRACT FROM AUTHOR]

Recent years have witnessed an increasing interest in time‐dependent coupled‐cluster (TDCC) theory for simulating laser‐driven electronic dynamics in atoms and molecules, and for simulating molecular vibrational dynamics. Starting from the time‐dependent bivariational principle, we review different flavors of single‐reference TDCC theory with either orthonormal static, orthonormal time‐dependent, or biorthonormal time‐dependent spin orbitals. The time‐dependent extension of equation‐of‐motion coupled‐cluster theory is also discussed, along with the applications of TDCC methods to the calculation of linear absorption spectra, linear and low‐order nonlinear response functions, highly nonlinear high harmonic generation spectra and ionization dynamics. In addition, the role of TDCC theory in finite‐temperature many‐body quantum mechanics is briefly described along with a few other application areas. This article is categorized under:Electronic Structure Theory > Ab Initio Electronic Structure MethodsTheoretical and Physical Chemistry > SpectroscopySoftware > Simulation Methods [ABSTRACT FROM AUTHOR]

While methodological developments in the last decade made it possible to compute coupled cluster (CC) energies including excitations up to a perturbative triples correction for molecules containing several hundred atoms, a similar breakthrough has not yet been reported for excited state computations. Accurate CC methods for excited states are still expensive, although some promising candidates for an efficient and accurate excited state CC method have emerged recently. This review examines the various approximation schemes with particular emphasis on their performance for excitation energies and summarizes the best state‐of‐the‐art results which may pave the way for a robust excited state method applicable to molecules of hundreds of atoms. Among these, special attention will be given to exploiting the techniques of similarity transformation, perturbative approximations as well as integral decomposition, local and embedding techniques within the equation of motion CC framework. This article is categorized under:Electronic Structure Theory > Ab Initio Electronic Structure MethodsStructure and Mechanism > Molecular Structures [ABSTRACT FROM AUTHOR]

Real-time time-dependent electronic structure theory is one of the most promising methods for investigating time-dependent molecular responses and electronic dynamics. Since its first modern use in the 1990s, it has been used to study a wide variety of spectroscopic properties and electronic responses to intense external electromagnetic fields, complex environments, and open quantum systems. It has also been used to study molecular conductance, excited state dynamics, ionization, and nonlinear optical effects. Real-time techniques describe non-perturbative responses of molecules, allowing for studies that go above and beyond the more traditional energy- or frequency-domain-based response theories. Recent progress in signal analysis, accurate treatment of environmental responses, relativistic Hamiltonians, and even quantized electromagnetic fields have opened up new avenues of research in time-dependent molecular response. After discussing the history of real-time methods, we explore some of the necessary mathematical theory behind the methods, and then survey a wide (yet incomplete) variety of applications for real-time methods. We then present some brief remarks on the future of real-time time-dependent electronic structure theory. WIREs Comput Mol Sci 2018, 8:e1341. doi: 10.1002/wcms.1341 This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods [ABSTRACT FROM AUTHOR]

Resummation methods can significantly improve the accuracy of ab initio electronic structure computations without increasing the computational cost. For perturbation theories, resummation methods can be designed by constructing approximants to model the known singularity structure of the theory in the complex plane of the perturbation parameter. Quadratic approximants for the fourth-order Møller-Plesset perturbation theory (MP4) greatly improve the accuracy for the ground-state energy and provide information about singularity positions that can be used to select an optimal summation method. The Coupled cluster theories CCSD (coupled clusters with single and double excitations), CCSDT (with triple excitations), CCSDTQ (with quadruple excitations), and CCSD(T) (with a triples correction from perturbation theory) can be resummed using approximants that model the empirically observed convergence patterns of the Hartree-Fock (HF), CCSD, CCSD(T) and HF, CCSD, CCSDT, CCSDTQ sequences. Coupling-constant perturbation theories of molecular vibration and of atoms in external fields, and semiclassical perturbation theories also benefit from appropriate approximants. © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Chronus Quantum (ChronusQ) software package is an open source (under the GNU General Public License v2) software infrastructure which targets the solution of challenging problems that arise in ab initio electronic structure theory. Special emphasis is placed on the consistent treatment of time dependence and spin in the electronic wave function, as well as the inclusion of relativistic effects in said treatments. In addition, ChronusQ provides support for the inclusion of uniform finite magnetic fields as external perturbations through the use of gauge‐including atomic orbitals. ChronusQ is a parallel electronic structure code written in modern C++ which utilizes both message passing implementation and shared memory (OpenMP) parallelism. In addition to the examination of the current state of code base itself, a discussion regarding ongoing developments and developer contributions will also be provided. This article is categorized under:Software > Quantum ChemistryElectronic Structure Theory > Ab Initio Electronic Structure MethodsElectronic Structure Theory > Density Functional Theory [ABSTRACT FROM AUTHOR]

We review the current state of reduced‐scaling electron correlation methods, particularly coupled‐cluster theory for the simulation and prediction of molecular response properties. The successes of local‐coupled‐cluster and related approaches are well known for reaction energies, thermodynamic constants, dipole moments, and so forth—properties that depend primarily on the quality of the ground‐state wave function. However, much more challenging are higher‐order properties such as polarizabilities, hyperpolarizabilities, optical rotations, magnetizabilities, and others that also require accurate representation of the derivative of the wave function to external electromagnetic fields. We discuss a range of methods for improving the correlation domains of such perturbed wave functions, including the use of "perturbation‐aware" natural orbitals that are customized for the property of interest. In addition, we consider the viability and potential of promising, but still‐emerging methods such as stochastic and real‐time coupled‐cluster approaches, for which the localizability of the field‐dependent wave function may be more controllable than for conventional response theory. This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure MethodsTheoretical and Physical Chemistry > SpectroscopySoftware > Quantum Chemistry [ABSTRACT FROM AUTHOR]