Coupled-cluster techniques for computational chemistry: The CFOUR program package

An up-to-date overview of the CFOUR program system is given. After providing a brief outline of the evolution of the program since its inception in 1989, a comprehensive presentation is given of its well-known capabilities for high-level coupled-cluster theory and its application to molecular properties. Subsequent to this generally well-known background information, much of the remaining content focuses on lesser-known capabilities of CFOUR, most of which have become available to the public only recently or will become available in the near future. Each of these new features is illustrated by a representative example, with additional discussion targeted to educating users as to classes of applications that are now enabled by these capabilities. Finally, some speculation about future directions is given, and the mode of distribution and support for CFOUR are outlined

VIB5 database with accurate ab initio quantum chemical molecular potential energy surfaces

High-level ab initio quantum chemical (QC) molecular potential energy surfaces (PESs) are crucial for accurately simulating molecular rotation-vibration spectra. Machine learning (ML) can help alleviate the cost of constructing such PESs, but requires access to the original ab initio PES data, namely potential energies computed on high-density grids of nuclear geometries. In this work, we present a new structured PES database called VIB5, which contains high-quality ab initio data on 5 small polyatomic molecules of astrophysical significance (CH3Cl, CH4, SiH4, CH3F, and NaOH). The VIB5 database is based on previously used PESs, which, however, are either publicly unavailable or lacking key information to make them suitable for ML applications. The VIB5 database provides tens of thousands of grid points for each molecule with theoretical best estimates of potential energies along with their constituent energy correction terms and a data-extraction script. In addition, new complementary QC calculations of energies and energy gradients have been performed to provide a consistent database, which, e.g., can be used for gradient-based ML methods

Pascual Jordan, his contributions to quantum mechanics and his legacy in contemporary local quantum physics

After recalling episodes from Pascual Jordan's biography including his pivotal role in the shaping of quantum field theory and his much criticized conduct during the NS regime, I draw attention to his presentation of the first phase of development of quantum field theory in a talk presented at the 1929 Kharkov conference. He starts by giving a comprehensive account of the beginnings of quantum theory, emphasising that particle-like properties arise as a consequence of treating wave-motions quantum-mechanically. He then goes on to his recent discovery of quantization of ``wave fields'' and problems of gauge invariance. The most surprising aspect of Jordan's presentation is however his strong belief that his field quantization is a transitory not yet optimal formulation of the principles underlying causal, local quantum physics. The expectation of a future more radical change coming from the main architect of field quantization already shortly after his discovery is certainly quite startling. I try to answer the question to what extent Jordan's 1929 expectations have been vindicated. The larger part of the present essay consists in arguing that Jordan's plea for a formulation without ``classical correspondence crutches'', i.e. for an intrinsic approach (which avoids classical fields altogether), is successfully addressed in past and recent publications on local quantum physics.Comment: More biographical detail, expansion of the part referring to Jordan's legacy in quantum field theory, 37 pages late

ADVANCING COUPLED CLUSTER METHODS: TENSOR FACTORIZATION AND ANALYTIC GRADIENT IMPLEMENTATION

With the rapid advancement of computing capabilities, computational chemistry has become increasingly indispensable in both experiment design and the interpretation of experimental outcomes. Wave-function-based quantum mechanistic methods are highly sought after for their ability to provide high-accuracy data and their potential for systematic improvement. However, to extend their applicability to larger molecules, it is essential to employ rank-reducing approximations to these methods. This dissertation dedicates Chapters Three and Four to the development of rank-reduced methodologies. Additionally, beyond single-point energies, molecular geometries and properties of molecule-excited states hold paramount importance across diverse fields of chemistry. Hence, Chapter Five presents the work on the implementation of the analytic gradient for the EOM-CCSD* method. Moreover, molecular mechanisms play an indispensable role, particularly in their scalability to biocomplexes. Chapter Six exemplifies this by demonstrating the advantageous application of molecular dynamics in exploring the protein allostery mechanism

Halide perovskites from first principles: From fundamental optoelectronic properties to the impact of structural and chemical heterogeneity

Organic-inorganic metal-halide perovskite semiconductors have outstanding and widely tunable optoelectronic properties suited for a broad variety of applications. First-principles numerical modelling techniques are playing a key role in unravelling structure-property relationships of this structurally and chemically diverse family of materials, and for predicting new materials and properties. Herein we review first-principles calculations of the photophysics of halide perovskites with a focus on the band structures, optical absorption spectra and excitons, and the effects of electron- and exciton-phonon coupling and temperature on these properties. We focus on first-principles approaches based on density functional theory and Green's function-based many-body perturbation theory and provide an overview of these approaches. While a large proportion of first-principles studies have been focusing on the prototypical ABX3 single perovskites based on Pb and Sn, recent years have witnessed significant efforts to further functionalize halide perovskites, broadening this family of materials to include double perovskites, quasi-low-dimensional structures, and other organic-inorganic materials, interfaces and heterostructures. While this enormous chemical space of perovskite and perovskite-like materials has only begun to be tapped experimentally, recent advances in theoretical and computational methods, as well as in computing infrastructure, have led to the possibility of understanding the photophysics of ever more complex systems. We illustrate this progress in our review by summarizing representative studies of first-principles calculations of halide perovskites with various degrees of complexity

Advanced capabilities for materials modelling with Quantum ESPRESSO

Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudo-potential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement theirs ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software