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Different Basis Sets (different + basis_set)
Selected AbstractsElectron localizability indicators ELI and ELIA: The case of highly correlated wavefunctions for the argon atomJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 8 2008Viktor Bezugly Abstract Electron localizability indicators based on the same-spin electron pair density and the opposite-spin electron pair density are studied for correlated wavefunctions of the argon atom. Different basis sets and reference spaces are used for the multireference configuration interaction method following the complete active space calculations aiming at the understanding of the effect of local electron correlation when approaching the exact wavefunction. The populations of the three atomic shells of Ar atom in real space are calculated for each case. © 2007 Wiley Periodicals, Inc. J Comput Chem 29: 1198,1207, 2008 [source] Gas-Phase Electron-Diffraction Investigation and Quantum-Chemical Calculations of the Structure of 1,5-Dimethylsemibullvalene-2,4,6,8-tetracarboxylic DianhydrideHELVETICA CHIMICA ACTA, Issue 5 2003Svein Samdal The bridged homotropilidines have been of interest for decades because their molecules offer the potential for homoaromaticity. Although many of these have been shown not to be homoaromatic, the energy differences of the delocalized (homoaromatic) forms and the localized (nonhomoaromatic) ones, and the barriers to the interconversion of the localized forms via a Cope rearrangement, have been found to vary greatly. The title compound is a strong candidate for homoaromaticity, and, since the structures of the possible localized and delocalized forms could differ significantly, we have carried out an electron-diffraction investigation of it augmented by quantum-mechanical calculations with different basis sets at several levels of theory. Three models were explored: one representing a localized form of Cs symmetry, one a delocalized form of C2v symmetry, and one a 2,:,1 mixture of the localized/delocalized forms. Although none of the models could be ruled out, the experimental evidence slightly favors the Cs form. These results are consistent with those from the DFT B3PW91 calculations with basis sets ranging from 6-31G(d) to cc-pVTZ, which, surprisingly, predict essentially equal thermally corrected free energies for each. The results are discussed. [source] Combined 13C NMR and DFT/GIAO studies of the polyketides Aurasperone A and Fonsecinone AINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2008Sergio de Lazaro Abstract Structural characterization by NMR spectroscopy and DFT calculations was performed for two dimeric naphtho-,-pyrones, the polyketides Aurasperone A and Fonsecinone A. Experimental data (13C NMR chemical shifts and interatomic geometries) were found to be in reasonable agreement with theoretical ones, obtained at B3LYP level for three different basis sets (6-31G/6-31G(d)/6-31G(d,p)). Additionally, the dipolar moments calculation allowed explaining the different solubility for these molecules. The 13C NMR theoretical chemical shifts were calculated with the GIAO method and the solvent effects were taken into account by means of the PCM approximation. In this work, the DFT/GIAO methodology shows to be a reliable tool in the assignment of experimental NMR chemical shifts of similar molecules. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] BSSE-free hardness profiles of hydrogen bond exchange in the hydrogen fluoride dimerINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2006David Asturiol Abstract Hardness profiles are calculated for the hydrogen bond interchange in the hydrogen fluoride dimer; the study is carried out at the Hartree,Fock and second-order Møller,Plesset (MP2) levels of theory with three different basis sets. The basis set superposition error (BSSE) is corrected by means of the chemical Hamiltonian approach (CHA). Uncorrected and BSSE-corrected energy and hardness profiles are compared. Their analysis shows that hardness profiles do not lead to spurious minima as energy profiles do in various cases. The CHA methodology is shown to be valuable in the analysis of intermolecular interactions, with BSSE observed to carry smaller modifications on hardness profiles than on energy curves. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [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] Intramolecular proton transfer induced by divalent alkali earth metal cation in the gas stateINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2003Hongqi Ai Abstract Interactions between divalent alkali earth metal (DAEM) ions M (MBe, Mg, Ca, Sr, Ba) and the second stable glycine conformer in the gas phase, which can transfer into the ground-state glycine-M2+ (except the glycine,Be2+) among each corresponding isomers when these divalent metal ions are bound, are studied at the hybrid three-parameter B3LYP level with three different basis sets. Proton transfers from the hydroxyl to the amino nitrogen of the glycine without energy barriers have been first observed in the gas phase in these glycine,M2+ systems. The interaction between the glycine and these DAEM ions except beryllium and magnesium ion only create an amino hydrogen pointing to the original hydroxyl due to their weaker interaction relative to those divalent transition metal (DTM) ion-bound glycine derivatives, being obviously different from that between the glycine and DTM ions, in which two amino hydrogens point to the original hydroxyl oxygen when these metal-chelated glycine derivatives are produced. The interaction energy between the glycine and divalent magnesium would be the boundary of one or two amino hydrogens pointing to the hydrogyl oxygen, i.e., the ,170.3 kcal/mol of binding energy is a critical point. Similar intramolecular proton transfer has also been predicted for those DTM ion-chelated glycine systems; however, that in the gas state has not been observed in the monovalent metal ion-coordinated glycine systems. The binding energy between some monovalent TM ion and the glycine is similar to that of the glycine,Ba2+, which has the lowest binding strength among these DAEM,ion chelated glycine complexes. The difference among them only lies in the larger electrostatic and polarized effects in the latter, which favor the stability of the zwitterionic glycine form in the gas phase. According to these observations, we predict that the zwitterionic glycine would exist in the field of two positive charges in the gas phase. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 205,214, 2003 [source] BSSE-free description of the formamide dimersINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2001A. Bende Abstract The different configurations (linear, zig-zag, and cyclic) of formamide dimers have been studied at the level of both Hartree,Fock (HF) and second order Møller,Plesset perturbation theory (MP2). The widely used a posteriori Boys,Bernardi "counterpoise" (CP) correction scheme has been compared with our a priori methods utilizing the "chemical Hamiltonian approach" (CHA). The appropriate interaction energies have been calculated in six different basis sets (6-31G, 6-31G**, DZV, DZP, TZV, and cc-pVDZ). © 2001 John Wiley & Sons, Inc. Int J Quant Chem, 2001 [source] Ab initio and DFT studies on van der Waals trimers: The OCS · (CO2)2 complexesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2002H. Valdés Abstract Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS · (CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm,1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 444,455, 2002; DOI 10.1002/jcc.10041 [source] Accurate prediction of proton chemical shifts.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2001Abstract Forty-five proton chemical shifts in 14 aromatic molecules have been calculated at several levels of theory: Hartree,Fock and density functional theory with several different basis sets, and also second-order Møller,Plesset (MP2) theory. To obtain consistent experimental data, the NMR spectra were remeasured on a 500 MHz spectrometer in CDCl3 solution. A set of 10 molecules without strong electron correlation effects was selected as the parametrization set. The calculated chemical shifts (relative to benzene) of 29 different protons in this set correlate very well with the experiment, and even better after linear regression. For this set, all methods perform roughly equally. The best agreement without linear regression is given by the B3LYP/TZVP method (rms deviation 0.060 ppm), although the best linear fit of the calculated shifts to experimental values is obtained for B3LYP/6-311++G**, with an rms deviation of only 0.037 ppm. Somewhat larger deviations were obtained for the second test set of 4 more difficult molecules: nitrobenzene, azulene, salicylaldehyde, and o -nitroaniline, characterized by strong electron correlation or resonance-assisted intramolecular hydrogen bonding. The results show that it is possible, at a reasonable cost, to calculate relative proton shieldings in a similar chemical environment to high accuracy. Our ultimate goal is to use calculated proton shifts to obtain constraints for local conformations in proteins; this requires a predictive accuracy of 0.1,0.2 ppm. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1887,1895, 2001 [source] Mechanism and dynamics of organic reactions: 1,2-H shift in methylchlorocarbene,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2002Elfi Kraka Abstract The unified reaction valley approach (URVA) was used to investigate the mechanism of the rearrangement of methylchlorocarbene to chloroethene [reaction(1)] in the gas phase with special emphasis on the role of H tunneling. The reaction valley of (1) was explored using different methods (HF, MP2 and DFT/B3LYP) and different basis sets [6,31G(d), 6,31G(d,p) and cc-pVTZ]. Results were analyzed characterizing normal modes, reaction path vector and curvature vector in terms of generalized adiabatic modes associated with internal parameters that are used to describe the reaction complex. For reaction (1), H tunneling plays a significant role even at room temperature, but does not explain the strongly curved Arrhenius correlations observed experimentally. The probability of H tunneling can be directly related to the curvature of the reaction path and the associated curvature couplings. The reaction is preceeded in the forward and reverse direction by energy-consuming conformational changes that prepare the reactant for the actual 1,2-H shift, which requires only little energy. The effective energy needed for CH bond breaking is just 6,kcal,mol,1 for (1). The gas-phase and the solution-phase mechanisms of (1) differ considerably, which is reflected by the activation enthalpies: 11.4 (gas, calculated) and 4.3,kcal,mol,1 (solution, measured). Stabilizing interactions with solvent molecules take place in the latter case and reduce the importance of H tunneling. The non-linearity of the measured Arrhenius correlations most likely results from bimolecular reactions of the carbene becoming more important at lower temperatures. Copyright © 2002 John Wiley & Sons, Ltd. [source] Appropriate SCF basis sets for orbital studies of galaxies and a ,quantum-mechanical' method to compute themMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008Constantinos Kalapotharakos ABSTRACT We address the question of an appropriate choice of basis functions for the self-consistent field (SCF) method of simulation of the N -body problem. Our criterion is based on a comparison of the orbits found in N -body realizations of analytical potential,density models of triaxial galaxies, in which the potential is fitted by the SCF method using a variety of basis sets, with those of the original models. Our tests refer to maximally triaxial Dehnen ,-models for values of , in the range 0 ,,, 1, i.e. from the harmonic core up to the weak cusp limit. When an N -body realization of a model is fitted by the SCF method, the choice of radial basis functions affects significantly the way the potential, forces or derivatives of the forces are reproduced, especially in the central regions of the system. We find that this results in serious discrepancies in the relative amounts of chaotic versus regular orbits, or in the distributions of the Lyapunov characteristic exponents, as found by different basis sets. Numerical tests include the Clutton-Brock and the Hernquist,Ostriker basis sets, as well as a family of numerical basis sets which are ,close' to the Hernquist,Ostriker basis set (according to a given definition of distance in the space of basis functions). The family of numerical basis sets is parametrized in terms of a quantity , which appears in the kernel functions of the Sturm,Liouville equation defining each basis set. The Hernquist,Ostriker basis set is the ,= 0 member of the family. We demonstrate that grid solutions of the Sturm,Liouville equation yielding numerical basis sets introduce large errors in the variational equations of motion. We propose a quantum-mechanical method of solution of the Sturm,Liouville equation which overcomes these errors. We finally give criteria for a choice of optimal value of , and calculate the latter as a function of the value of ,, i.e. of the power-law exponent of the radial density profile at the central regions of the galaxy. [source] | ||||||||||