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.

Quantum ESPRESSO is an open-source distribution of computer codes for quantum-mechanical materials modeling, based on density-functional theory, pseudopotentials, and plane waves, and renowned for its performance on a wide range of hardware architectures, from laptops to massively parallel computers, as well as for the breadth of its applications. In this paper, we present a motivation and brief review of the ongoing effort to port Quantum ESPRESSO onto heterogeneous architectures based on hardware accelerators, which will overcome the energy constraints that are currently hindering the way toward exascale computing.

The subtle interplay of several different effects makes the interpretation and analysis of experimental spectra in terms of structural and dynamic characteristics a very challenging task. In this context, theoretical studies can be very helpful, and this is the reason behind the rapid evolution of computational spectroscopy from a highly specialized research field toward a versatile and widespread tool. However, in the case of vibrational spectra of large molecular systems, the most popular approach still relies on a harmonic treatment, because of the difficulty to explore the multidimensional anharmonic potential energy surface. These can be overcome considering that, in many cases, the vibrational transitions are well localized and only some of them are observed experimentally. To this aim, the procedure for the simulation of vibrational spectra of large molecular systems beyond the harmonic approximation is discussed. The quality of system-specific reduced dimensional anharmonic approaches is first validated by comparison with computations taking into account all modes simultaneously for anisole and glycine. Next, the approach is applied to two larger systems, namely glycine adsorbed on a silicon surface and chlorophyll-a in solution, and the results are compared with experimental data showing significant improvement over the standard harmonic approximation. Our results show that properly tailored reduced dimension anharmonic approaches stand as feasible routes for state-of-the-art computational spectroscopy studies and allow to take into account both anharmonic and environmental effects on the spectra even for relatively large molecular systems.

The recent implementation of the computation of IR intensities beyond the double-harmonic approximation [Bloino, J.; Barone, V. J. Chem. Phys. 2012, 136, 124108] paved the route to routine calculations of infrared spectra for a wide set of molecular systems. Halogenated organic compounds represent an interesting class of molecules, from both an atmospheric and computational point of view, due to the peculiar chemical features related to the halogen atoms. In this work we simulate the IR spectra of eight halogenated molecules (CH 2 F 2 , CHBrF 2 , CH 2 DBr, CF 3 Br, CH 2 CHF, CF 2 CFCl, cis-CHFCHBr, cis-CHFCHI), using two common hybrid and doublehybrid density functionals in conjunction with both double-and triple-zeta quality basis sets (SNSD and cc-pVTZ) as well as employing the coupled-cluster theory with basis sets of at least triple-zeta quality. Finally, we compare our results with available experimental spectra, with the aim of checking the accuracy and the performances of the computational approaches.

A new formulation of time-dependent density functional tight binding (TD-DFTB) is reported in this paper. It is derived from the application of the linear response theory to the ground state DFTB Hamiltonian, without the introduction of additional parameters for the description of the excited states. The method is validated for several sets of organic compounds, against the best theoretical estimates from the literature, density functional theory, semiempirical methods, and experimental data. The comparison shows that TD-DFTB gives reliable results both for singlet and triplet excitation energies. In addition, the application of TD-DFTB to open-shell systems shows promising results.

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The Density Functional Tight Binding (DFTB) and Time Dependent DFTB (TD-DFTB) methods have been coupled with the Polarizable Continuum Model (PCM) of solvation, aiming to study spectroscopic properties for large systems in condensed phases. The calculation of the ground and the excited state energies, together with the analytical gradient and Hessian of the ground state energy, have been implemented in a fully analytical and computationally effective approach. After sketching the theoretical background of both DFTB and PCM, we describe the details of both the formalism and the implementation. We report a number of examples ranging from vibrational to electronic spectroscopy, and we identify the strengths and the limitations of the DFTB/PCM method. We also evaluate DFTB as a component in a hybrid approach, together with a more refined quantum mechanical (QM) method and PCM, for the specific case of anharmonic vibrational spectra.

In this paper, we extend an integrated QM/MM/polarizable continuum model (PCM) method, which combines a fluctuating charge (FQ) approach to the MM polarization with the PCM, to describe electronic circular dichroism (ECD) spectra of systems in aqueous solution. The main features of the approach are presented, and then applications to the UV and ECD spectra of neutral (S)-nicotine in aqueous solution are reported. The performance of the QM/FQ/PCM is compared with that of the PCM against newly measured UV ECD spectra, which are in agreement with previous findings. The inclusion of specific solvation effects via the QM/FQ/PCM method leads to an improvement in the calculated spectra compared to the experimental findings, though the pure PCM results are still qualitatively correct and are a useful tool for the characterization of the states.