Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design

A spin-flip variant of the second-order approximate coupled-cluster singles and doubles method

We report an implementation of a spin-flip variant of the second-order approximate coupled-cluster singles and doubles (CC2) method. The resolution-of-the-identity approximation or, alternatively, Cholesky decomposition of the two-electron integrals are used to reduce the memory requirements. We illustrate the performance of the new method by constructing potential energy curves of H2 and HF and by computing singlet-triplet splittings for various diradicals including some binuclear copper complexes that are of interest as molecular magnets. We find that spin-flip CC2 performs very similarly to the spin-flip variant of the algebraic diagrammatic construction scheme for the polarization propagator of second order (ADC(2)). Application to ozone shows that spin-flip CC2 predicts a barrierless symmetric dissociation of this molecule similar to spin-conserving CC2 and in contrast to spin-flip ADC(2) and coupled-cluster singles and doubles

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design