1,720,976 research outputs found
Analytical gradients for MP2, double hybrid functionals, and TD-DFT with polarizable embedding described by fluctuating charges
Full text linkA polarizable quantum mechanics (QM)/ molecular mechanics (MM) approach recently developed for Hartree-Fock (HF) and Kohn-Sham (KS) methods has been extended to energies and analytical gradients for MP2, double hybrid functionals, and TD-DFT models, thus allowing the computation of equilibrium structures for excited electronic states together with more accurate results for ground electronic states. After a detailed presentation of the theoretical background and of some implementation details, a number of test cases are analyzed to show that the polarizable embedding model based on fluctuating charges (FQ) is remarkably more accurate than the corresponding electronic embedding based on a fixed charge (FX) description. In particular, a set of electronegativities and hardnesses has been optimized for interactions between QM and FQ regions together with new repulsion-dispersion parameters. After validation of both the numerical implementation and of the new parameters, absorption electronic spectra have been computed for representative model systems including vibronic effects. The results show remarkable agreement with full QM computations and significant improvement with respect to the corresponding FX results. The last part of the article provides some hints about computation of solvatochromic effects on absorption spectra in aqueous solution as a function of the number of FQ water molecules and on the use of FX external shells to improve the convergence of the results. © 2015 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc
Computer simulations of prebiotic systems
No full textComputer simulations are powerful tools in order to figure out the physical and chemical phenomena occurring in the prebiotic environments, shedding light on the processes that led to the emergence of life on Earth. Indeed, the state-of-the-art theoretical methods have been developed for predicting molecular properties of the isolated systems and for describing the intermolecular interactions and the reactivity in a wide range of environments and boundary conditions. Such studies strongly help to unravel the various molecular contributions to the intricate experimental outcomes, allowing a deeper understanding of the underlying phenomena. This contribution provides an overview of the theoretical studies of prebiotic systems, drawing attention to some application of specific computational tool
Computational Tools for Structure, Spectroscopy and Thermochemistry
No full textIn this chapter we will review the main methodological aspects providing the background for the computational study of thermochemical and spectroscopic properties of molecular systems. A variety of spectroscopies covering a large interval of the electromagnetic spectrum, from the radiofrequencies to the UV-Vis zones, have been applied to a selected set of molecular systems of interest in organometallic chemistry. Both isolated molecules in the gas and condensed phases and nanosystems have been studied, giving particular emphasis to the interplay between experiment and theory: stereoelectronic, vibrational, vibronic, and environmental effects are discussed for few illustrative examples. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved
Fast hybrid density-functional computations using plane-wave basis sets
Full text linkA new, very fast, implementation of the exact (Fock) exchange operator for electronic-structure calculations within the plane-wave pseudopotential method is described and carefully validated. Our method combines the recently proposed adaptively compressed exchange approach, to reduce the number of times the exchange is evaluated in the self-consistent loop, with an orbital localization procedure that reduces the number of exchange integrals to be computed at each evaluation. The new implementation, already available in the Q uantum ESPRESSO distribution, results in a speedup that is never smaller than 3–4 × and that increases with the size of the system, according to various realistic benchmark calculations
Fully ab initio IR spectra for complex molecular systems from perturbative vibrational approaches: Glycine as a test case
No full textPerturbative anharmonic computations have been used to simulate the IR spectrum of glycine, taking into account its three most stable conformers. The theoretical results have been directly compared with their experimental counterparts, showing good agreement between the latter and the spectra obtained after proper averaging of the contributions from the three most stable glycine conformers. The results show that direct simulation of the overall vibrational spectrum within a second-order perturbative treatment is feasible and leads to a better understanding of experimental data. Additionally, it has been shown that accurate results can be obtained even when several molecular species need to be considered simultaneously. The computations performed at the B3LYP/aug-N07D level have shown their reliability in the prediction of both vibrational energy levels and IR intensities beyond the harmonic approximation. This kind of computations represents an important tool for the analysis of vibrational spectra for complex medium-to-large molecular systems. (C) 2011 Elsevier B.V. All rights reserved
Development, validation, and pilot application of a generalized fluctuating charge model for computational spectroscopy in solution
Full text linkA general approach enforcing nonperiodic boundary conditions for the computation of spectroscopic properties in solution has been improved including an effective description of charge-transfer contributions and coordination number adjustment for explicit solvent molecules. Both contributions are obtained from a continuous description of intermolecular hydrogen bonds, which has been employed also for an effective clustering of molecular dynamics trajectories. Fine tuning of the model has been performed for several water clusters, and then its efficiency and reliability have been demonstrated by computing the absorption spectra of different creatinine tautomers in aqueous solution
Optical rotatory dispersion of methyloxirane in aqueous solution: assessing the performance of density functional theory in combination with a fully polarizable QM/MM/PCM approach
Full text linkWe report a study on the performance of a recently developed
fully polarizable QM/MM/PCM approach based on Fluctuating Charges
(FQ) combined with 11 different Density Functionals for the description
of the Optical Rotation at different wavelengths of (R)-Methyloxirane in
aqueous solution. The results are compared with those obtained for the
isolated system and for the solvated one as described by the Polarizable
Continuum Model. In all cases, a comparison with experimental data is
also shown. The results show that the effect of the solvent is much more
significant than the effect of the density functional
Computational Spectroscopy of Large Systems in Solution: The DFTB/PCM and TD-DFTB/PCM Approach
No full textThe 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
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