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Diffuse Functions (diffuse + function)
Selected AbstractsQuantitative assessment of the effect of basis set superposition error on the electron density of molecular complexes by means of quantum molecular similarity measuresINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 11 2009Pedro Salvador Abstract The Chemical Hamiltonian Approach (CHA) method is applied to obtain Basis Set Superposition Error (BSSE)-free molecular orbitals at the Hartree,Fock (HF) and Density Functional Theory (DFT) levels of theory. To assess qualitatively the effect of the BSSE on the first-order electron density, we had previously applied Bader's analysis of the intermolecular critical points located on the electron density, as well as density difference maps for several hydrogen bonded complexes. In this work, Quantum Molecular Similarity Measures are probed as an alternative avenue to properly quantify the electronic relaxation due to the BSSE removal by means of distance indices between the uncorrected and corrected charge densities. It is shown that BSSE contamination is more important at the DFT level of theory, and in some cases, changes on the topology of the electron density are observed upon BSSE correction. Inclusion of diffuse functions have been found to dramatically decrease the BSSE effect in both geometry and electron density. The CHA method represents a good compromise to obtain accurate results with small basis sets. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Complete basis set extrapolations of dispersion, exchange, and coupled-clusters contributions to the interaction energy: a helium dimer study,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 12 2008gorzata Jeziorska Abstract Effectiveness of various extrapolation schemes in predicting complete basis set (CBS) values of interaction energies has been investigated for the helium dimer as a function of interatomic separation R. The investigations were performed separately for the leading dispersion and exchange contributions to the interaction energy and for the interaction energy computed using the coupled cluster method with single and double excitations (CCSD). For all these contributions, practically exact reference values were obtained from Gaussian-type geminal calculations. Sequences of orbital basis sets augmented with diffuse and bond functions or augmented with two sets of diffuse functions have been employed, with the cardinal numbers up to X = 7. The functional form EX = ECBS + A(X , k),, was applied for the extrapolations, where EX is the contribution to the interaction energy computed with a basis set of cardinal number X. The main conclusion of this work is that CBS extrapolations of an appropriate functional form generally improve the accuracy of the interaction energies at a very small additional computational cost (of the order of 10%) and should be recommended in calculations of interatomic and intermolecular potentials. The effectiveness of the extrapolations significantly depends, however, on the interatomic separation R and on the composition of the basis set. Basis sets with midbond functions, well known to provide at a given size much more accurate nonextrapolated results than bases lacking such functions, have been found to perform best also in extrapolations. The X,1 extrapolations of dispersion energies computed with midbond function turned out to be very efficient (except at large R), reducing the errors by an order of magnitude for small X and a factor of two for large X (where the errors of nonextrapolated results are already very small). If midbond functions are not used, the X,3 formula is most appropriate for the dispersion energies. For the exchange component of the interaction energy, the best results are obtained,in both types of basis sets,with the X,4 extrapolation, which leads (in both cases) to almost an order of magnitude reduction of the error. The X,3 and (X , 1),3 extrapolations work also well, but give smaller improvements. The correlation component of the CCSD interaction energy extrapolates best with , between 2 and 3 for bases with midbond functions and between 3 and 4 for bases without such functions. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Principal component analysis of the effects of wavefunction modification on the electrostatic potential of indoleINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2005Maíra A. Carvalho Abstract The molecular electrostatic potential (MEP) of the indole molecule was calculated in a three-dimensional grid in which the molecule was centered at the origin. To evaluate the dependence of MEP on the type of calculation, semiempirical, ab initio, and density functional theory methods with different basis sets were employed. The data matrix generated by these calculations was analyzed by principal component analysis (PCA). The appearance of outliers and the effect of wavefunction modifications such as the introduction of electron correlations and diffuse functions were highlighted by the use of PCA. The spatial localization of such effects around the molecule was possible from the loadings values associated with the graphical analysis of the grid points. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Study of electronic spectra of free-base porphin and Mg-porphin: Comprehensive comparison of variety of ab initio, DFT, and semiempirical methodsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2005Josef Abstract SAC (symmetry adapted cluster)/SAC-CI and CASPT2 (multiconfigurational second-order perturbation theory) electron excitation spectra of free-base porphin and magnesium-porphin were determined using basis set functions augmented by both the polarization and diffuse functions,6-31+G(d). Such basis is recommended for correct description of the spectra because diffuse functions play fundamental roles in the formation of Rydberg MOs. The obtained results indicated that already the lowest roots in Au, B1u, B2g, and B3g irreducible representations display Rydberg character. The calculated spectra are in a good agreement with both experimental and recently calculated electronic transitions. It is concluded that the SAC/SAC-CI level spectral lines are significantly affected by configuration selection when energy thresholds 5.0 × 10,6 and 5.0 × 10,7 a.u. are used for the determination of ground and excited state properties. © 2004 Wiley Periodicals, Inc. J Comput Chem 26: 294,303, 2005 [source] Molecules for materials: Germanium hydride neutrals and anions.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2002Ge2Hn/Ge2H (n = 0, Molecular structures, electron affinities, thermochemistry of GeHn/GeH (n = 0 Abstract The GeHn (n = 0,4) and Ge2Hn (n = 0,6) systems have been studied systematically by five different density functional methods. The basis sets employed are of double-, plus polarization quality with additional s- and p-type diffuse functions, labeled DZP++. For each compound plausible energetically low-lying structures were optimized. The methods used have been calibrated against a comprehensive tabulation of experimental electron affinities (Chemical Reviews 102, 231, 2002). The geometries predicted in this work include yet unknown anionic species, such as Ge2H,, Ge2H, Ge2H, Ge2H, and Ge2H. In general, the BHLYP method predicts the geometries closest to the few available experimental structures. A number of structures rather different from the analogous well-characterized hydrocarbon radicals and anions are predicted. For example, a vinylidene-like GeGeH structure is the global minimum of Ge2H. For neutral Ge2H4, a methylcarbene-like HGë-GeH3 is neally degenerate with the trans -bent H2GeGeH2 structure. For the Ge2H anion, the methylcarbene-like system is the global minimum. The three different neutral-anion energy differences reported in this research are: the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). For this family of molecules the B3LYP method appears to predict the most reliable electron affinities. The adiabatic electron affinities after the ZPVE correction are predicted to be 2.02 (Ge2), 2.05 (Ge2H), 1.25 (Ge2H2), 2.09 (Ge2H3), 1.71 (Ge2H4), 2.17 (Ge2H5), and ,0.02 (Ge2H6) eV. We also reported the dissociation energies for the GeHn (n = 1,4) and Ge2Hn (n = 1,6) systems, as well as those for their anionic counterparts. Our theoretical predictions provide strong motivation for the further experimental study of these important germanium hydrides. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1642,1655, 2002 [source] Raman and surface-enhanced Raman spectra of flavone and several hydroxy derivativesJOURNAL OF RAMAN SPECTROSCOPY, Issue 7 2007Tatyana Teslova Abstract The Raman and surface-enhanced Raman spectra (SERS) of flavone and three of its hydroxy derivatives, 3-hydroxyflavone (3-HF) and 5-hydroxyflavone (5-HF) and quercetin (3,5,7,3,,4, pentahydroxyflavone) have been obtained. The normal Raman (NR) spectra were taken in the powder form. The SERS spectra were obtained both on Ag colloids and Ag electrode substrates. Assignments of the spectrally observed normal modes were aided by density functional theory (DFT) calculations using the B3LYP functional and the 6-31 + G* basis, a split valence polarized basis set with diffuse functions. Excellent fits were obtained for the observed spectra with little or no scaling. The most intense lines of the NR spectra are those in the CO stretching region (near 1600 cm,1). These lines are often weakened by proximity to the surface, while other lines at lower wavenumbers, due to in-plane ring stretches, tend to be strongly enhanced. The SERS spectrum of flavone is weak both on the colloid and on the electrode, indicating weak attachment to the surface. In contrast, the SERS spectra of the hydroxy derivatives of flavone are intense, indicating the assistance of OH groups in attachment to the surface. The spectra of the various species are compared, and a case study of application to detection of a textile dye (Persian berries), which contains quercetin, is presented. Copyright © 2007 John Wiley & Sons, Ltd. [source] Theoretical Simulation of Vibrational Sum-Frequency Generation Spectra from Density Functional Theory: Application to p -Nitrothiophenol and 2,4-DinitroanilineCHEMPHYSCHEM, Issue 12 2009Julien Guthmuller Dr. Abstract The molecular orientation of adsorbed molecules forming self-assembled monolayers can be determined by combining vibrational sum-frequency generation (SFG) measurements with quantum chemical calculations. Herein, we present a theoretical methodology used to simulate the SFG spectra for different combinations of polarizations. These simulations are based on calculations of the IR vectors and Raman tensors, which are obtained from density functional theory computations. The dependency of the SFG vibrational signature with respect to the molecular orientation is presented for the molecules p -nitrothiophenol and 2,4-dinitroaniline. It is found that a suitable choice of basis set as well as of exchange-correlation (XC) functional is mandatory to correctly simulate the SFG intensities and consequently provide an accurate estimation of the adsorbed molecule orientation. Comparison with experimental data shows that calculations performed at the B3LYP/6-311++G(d,p) level of approximation provide good agreement with experimental frequencies, and with IR and Raman intensities. In particular, it is demonstrated that polarization and diffuse functions are compulsory for reproducing the IR and Raman spectra, and consequently vibrational SFG spectra, of systems such as p -nitrothiophenol. Moreover, the investigated XC functionals reveal their influence on the relative intensities, which show rather systematic variations with the amount of Hartree,Fock exchange. Finally, further aspects of the modeling are revealed by considering the frequency dependence of the Raman tensors. [source] Ab initio prediction of optical rotation: Comparison of density functional theory and Hartree-Fock methods for three 2,7,8-trioxabicyclo[3.2.1]octanesCHIRALITY, Issue 4 2002P.J. Stephens Abstract We report ab initio calculations of the frequency-dependent electric dipole-magnetic dipole polarizabilities, ,(,), at the sodium D line frequency and, thence, of the specific rotations, [,]D, of 2,7,8-trioxabicyclo[3.2.1]octane, 1, and its 1-methyl derivative, 2, using the Density Functional Theory (DFT) and Hartree-Fock/Self-Consistent Field (HF/SCF) methodologies. Gauge-invariant (including) atomic orbitals (GIAOs) are used to ensure origin-independent [,]D values. Using large basis sets which include diffuse functions DFT [,]D values are in good agreement with experimental values (175.8° and 139.2° for (1S,5R)- 1 and - 2, respectively); errors are in the range 25,35°. HF/SCF [,]D values, in contrast, are much less accurate; errors are in the range 75,95°. The use of small basis sets which do not include diffuse functions substantially lowers the accuracy of predicted [,]D values, as does the use of the static limit approximation: ,(,) , ,(o). The use of magnetic-field-independent atomic orbitals, FIAOs, instead of GIAOs, leads to origin-dependent, and therefore nonphysical, [,]D values. We also report DFT calculations of [,]D for the 1-phenyl derivative of 1, 3. DFT calculations find two stable conformations, differing in the orientation of the phenyl group, of very similar energy, and separated by low barriers. Values of [,]D predicted using two different algorithms for averaging over phenyl group orientations are in good agreement with experiment. In principle, the absolute configuration (AC) of a chiral molecule can be assigned by comparison of the optical rotation predicted ab initio to the experimental value. Our results demonstrate the critical importance of the choice of ab initio methodology in obtaining reliable optical rotations and, hence, ACs, and show that, at the present time, DFT constitutes the method of choice. Chirality 14:288,296, 2002. © 2002 Wiley-Liss, Inc. [source] | ||||||||||