2,572 research outputs found
Interface of the polarizable continuum model of solvation with semi-empirical methods in the GAMESS program
An interface between semi-empirical methods and the polarized continuum model
(PCM) of solvation successfully implemented into GAMESS following the approach
by Chudinov et al (Chem. Phys. 1992, 160, 41). The interface includes energy
gradients and is parallelized. For large molecules such as ubiquitin a
reasonable speedup (up to a factor of six) is observed for up to 16 cores. The
SCF convergence is greatly improved by PCM for proteins compared to the gas
phase
Revised self-consistent continuum solvation in electronic-structure calculations
The solvation model proposed by Fattebert and Gygi [Journal of Computational
Chemistry 23, 662 (2002)] and Scherlis et al. [Journal of Chemical Physics 124,
074103 (2006)] is reformulated, overcoming some of the numerical limitations
encountered and extending its range of applicability. We first recast the
problem in terms of induced polarization charges that act as a direct mapping
of the self-consistent continuum dielectric; this allows to define a functional
form for the dielectric that is well behaved both in the high-density region of
the nuclear charges and in the low-density region where the electronic
wavefunctions decay into the solvent. Second, we outline an iterative procedure
to solve the Poisson equation for the quantum fragment embedded in the solvent
that does not require multi-grid algorithms, is trivially parallel, and can be
applied to any Bravais crystallographic system. Last, we capture some of the
non-electrostatic or cavitation terms via a combined use of the quantum volume
and quantum surface [Physical Review Letters 94, 145501 (2005)] of the solute.
The resulting self-consistent continuum solvation (SCCS) model provides a very
effective and compact fit of computational and experimental data, whereby the
static dielectric constant of the solvent and one parameter allow to fit the
electrostatic energy provided by the PCM model with a mean absolute error of
0.3 kcal/mol on a set of 240 neutral solutes. Two parameters allow to fit
experimental solvation energies on the same set with a mean absolute error of
1.3 kcal/mol. A detailed analysis of these results, broken down along different
classes of chemical compounds, shows that several classes of organic compounds
display very high accuracy, with solvation energies in error of 0.3-0.4
kcal/mol, whereby larger discrepancies are mostly limited to self-dissociating
species and strong hydrogen-bond forming compounds.Comment: The following article has been accepted by The Journal of Chemical
Physics. After it is published, it will be found at
http://link.aip.org/link/?jcp
Solvent effects on the properties of hyperbranched polythiophenes
The structural and electronic properties of all-thiophene dendrimers and dendrons in solution have been evaluated using very different theoretical approaches based on quantum mechanical (QM) and hybrid QM/molecular mechanics (MM) methodologies: (i) calculations on minimum energy conformations using an implicit solvation model in combination with density functional theory (DFT) or time-dependent DFT (TD-DFT) methods; (ii) hybrid QM/MM calculations, in which the solute and solvent molecules are represented at the DFT level as point charges, respectively, on snapshots extracted from classical molecular dynamics (MD) simulations using explicit solvent molecules, and (iii) QM/MM-MD trajectories in which the solute is described at the DFT or TD-DFT level and the explicit solvent molecules are represented using classical force-fields. Calculations have been performed in dichloromethane, tetrahydrofuran and dimethylformamide. A comparison of the results obtained using the different approaches with the available experimental data indicates that the incorporation of effects associated with both the conformational dynamics of the dendrimer and the explicit solvent molecules is strictly necessary to satisfactorily reproduce the properties of the investigated systems. Accordingly, QM/MM-MD simulations are able to capture such effects providing a reliable description of electronic properties–conformational flexibility relationships in all-Th dendrimers.Peer ReviewedPostprint (author's final draft
Two\u27s Company, Three\u27s a Crowd: Exciton Localization in Cofacially Arrayed Polyfluorenes
Understanding the mechanisms of long-range energy transfer through polychromophoric assemblies is critically important in photovoltaics and biochemical systems. Using a set of cofacially arrayed polyfluorenes (Fn), we investigate the mechanism of (singlet) exciton delocalization in π-stacked polychromophoric assemblies. Calculations reveal that effective stabilization of an excimeric state requires an ideal sandwich-like arrangement; yet surprisingly, emission spectroscopy indicates that exciton delocalization is limited to only two fluorene units for all n. Herein, we show that delocalization is determined by the interplay between the energetic gain from delocalization, which quickly saturates beyond two units in larger Fn, and an energetic penalty associated with structural reorganization, which increases linearly with n. With these insights, we propose a hopping mechanism for exciton transfer, based upon the presence of multiple excimeric tautomers of similar energy in larger polyfluorenes (n ≥ 4) together with the anticipated low thermal barrier of their interconversion
Range separated hybrid density functional study of organic dye sensitizers on anatase TiO nanowires
The adsorption of organic molecules coumarin and the donor--acceptor
type tetrahydroquinoline (C2-1) on anatase (101) and (001) nanowires have been
investigated using screened Coulomb hybrid density functional theory
calculations. While coumarin forms single bond with the nanowire surface, C2-1
additionally exhibits bidentate mode giving rise to much stronger adsorption
energies. Nonlinear solvation effects on the binding characteristics of the dye
chromophores on the nanowire facets have also been examined. These two dye
sensitizers show different electronic charge distributions for the highest
occupied and the lowest unoccupied molecular states. We studied the electronic
structures in terms of the positions of the band edges and adsorbate related
band gap states and their effect on the absorption spectra of the dye-nanowire
combined systems. These findings were interpreted and discussed from the view
point of better light harvesting and charge separation as well as in relation
to more efficient charge carrier injection into the semiconductor nanowire.Comment: 8 pages, 4 figures, and 1 tabl
Theoretical studies of 31P NMR spectral properties of phosphanes and related compounds in solution
Selected theoretical methods, basis sets and solvation models have been tested in their ability to predict 31P NMR chemical shifts of large phosphorous-containing molecular systems in solution. The most efficient strategy was found to involve NMR shift calculations at the GIAO-MPW1K/6-311++G(2d,2p)//MPW1K/6-31G(d) level in combination with a dual solvation model including the explicit consideration of single solvent molecules and a continuum (PCM) solvation model. For larger systems it has also been established that reliable 31P shift predictions require Boltzmann averaging over all accessible conformations in solution
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