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Basis Set Limit (basis + set_limit)
Selected AbstractsGround states of BeC and MgC: A comparative multireference Brillouin,Wigner coupled cluster and configuration interaction study,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005Vasilios I. Teberekidis Abstract The competing X3,, and 5,, states of the experimentally unknown alkaline,earth metal carbides BeC and MgC are examined with the multireference Brillouin,Wigner coupled cluster method restricted to single and double excitations (MRBW-CCSD). The results are compared against the traditional single-reference CCSD approach, as well as with other single and multireference methods. In both molecules, the CCSD 5,, , X3,, energy difference is underestimated, leading to an "erroneous" ground-state prediction in BeC. The MRBW-CCSD method corrects this anomalous behavior, leading to fair agreement with multireference configuration interaction (MR-CI) predictions. Our results at the highest levels of theory are extrapolated to the basis set limit, and the core/valence correlation is taken into account, leading to very accurate energetics and spectroscopic constants in both carbides. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] High-level ab initio calculations on HGeCl and the equilibrium geometry of the Ã1A, state derived from Franck-Condon analysis of the single-vibronic-level emission spectra of HGeCl and DGeClJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2010Daniel K. W. Mok Abstract CCSD(T) and/or CASSCF/MRCI calculations have been carried out on the X,1A, and Ã1A, states of HGeCl. The fully relativistic effective core potential, ECP10MDF, and associated standard valence basis sets of up to the aug-cc-pV5Z quality were employed for Ge. Contributions from core correlation and extrapolation to the complete basis set limit were included in determining the computed equilibrium geometrical parameters and relative electronic energy of these two states of HGeCl. Based on the currently, most systematic CCSD(T) calculations performed in this study, the best theoretical geometrical parameters of the X,1A, state are re(HGe) = 1.580 ± 0.001 Å, ,e = 93.88 ± 0.01° and re(GeCl) = 2.170 ± 0.001 Å. In addition, Franck-Condon factors including allowance for anharmonicity and Duschinsky rotation between these two states of HGeCl and DGeCl were calculated employing CCSD(T) and CASSCF/MRCI potential energy functions, and were used to simulate Ã1A, , X,1A, SVL emission spectra of HGeCl and DGeCl. The iterative Franck-Condon analysis (IFCA) procedure was carried out to determine the equilibrium geometrical parameters of the Ã1A, state of HGeCl by matching the simulated, and available experimental SVL emission spectra of HGeCl and DGeCl of Tackett et al., J Chem Phys 2006, 124, 124320, using the available, estimated experimental equilibrium (r) structure for the X,1A, state, while varying the equilibrium geometrical parameters of the Ã1A, state systematically. Employing the derived IFCA geometry of re(HGe) = 1.590 Å, re(GeCl) = 2.155 Å and ,e(HGeCl) = 112.7° for the Ã1A, state of HGeCl in the spectral simulation, the simulated absorption and SVL emission spectra of HGeCl and DGeCl agree very well with the available experimental LIF and SVL emission spectra, respectively. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Energy landscapes of nucleophilic substitution reactions: A comparison of density functional theory and coupled cluster methodsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2007Marcel Swart Abstract We have carried out a detailed evaluation of the performance of all classes of density functional theory (DFT) for describing the potential energy surface (PES) of a wide range of nucleophilic substitution (SN2) reactions involving, amongst others, nucleophilic attack at carbon, nitrogen, silicon, and sulfur. In particular, we investigate the ability of the local density approximation (LDA), generalized gradient approximation (GGA), meta-GGA as well as hybrid DFT to reproduce high-level coupled cluster (CCSD(T)) benchmarks that are close to the basis set limit. The most accurate GGA, meta-GGA, and hybrid functionals yield mean absolute deviations of about 2 kcal/mol relative to the coupled cluster data, for reactant complexation, central barriers, overall barriers as well as reaction energies. For the three nonlocal DFT classes, the best functionals are found to be OPBE (GGA), OLAP3 (meta-GGA), and mPBE0KCIS (hybrid DFT). The popular B3LYP functional is not bad but performs significantly worse than the best GGA functionals. Furthermore, we have compared the geometries from several density functionals with the reference CCSD(T) data. The same GGA functionals that perform best for the energies (OPBE, OLYP), also perform best for the geometries with average absolute deviations in bond lengths of 0.06 Å and 0.6°, even better than the best meta-GGA and hybrid functionals. In view of the reduced computational effort of GGAs with respect to meta-GGAs and hybrid functionals, let alone coupled cluster, we recommend the use of accurate GGAs such as OPBE or OLYP for the study of SN2 reactions. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] Ab initio investigation on the reaction path and rate for the gas-phase reaction of HO + H2O , H2O + OHJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2003Tadafumi Uchimaru Abstract This article describes an ab initio investigation on the potential surfaces for one of the simplest hydrogen atom abstraction reactions, that is, HO + H2O , H2O + OH. In accord with the findings in the previously reported theoretical investigations, two types of the hydrogen-bonding complexes [HOHOH] and [H2OHO] were located on the potential energy surface. The water molecule acts as a hydrogen donor in the [HOHOH] complex, while the OH radical acts as a hydrogen donor in the [H2OHO] complex. The energy evaluations at the MP2(FC) basis set limit, as well as those through the CBS-APNO procedure, have provided estimates for enthalpies of association for these complexes at 298 K as ,2.1 , ,2.3 and ,4.1 , ,4.3 kcal/mol, respectively. The IRC calculations have suggested that the [H2OHO] complex should be located along the reaction coordinate for the hydrogen abstraction. Our best estimate for the classical barrier height for the hydrogen abstraction is 7.8 kcal/mol, which was obtained from the CBS-APNO energy evaluations. After fitting the CBS-APNO potential energy curve to a symmetrical Eckart function, the rate constants were calculated by using the transition state theory including the tunneling correction. Our estimates for the Arrhenius parameters in the temperature region from 300 to 420 K show quite reasonable agreement with the experimentally derived values. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1538,1548, 2003 [source] Complete basis set prediction of methanol isotropic nuclear magnetic shieldings and indirect nuclear spin,spin coupling constants (SSCC) using polarization-consistent and XZP basis sets and B3LYP and BHandH density functionalsMAGNETIC RESONANCE IN CHEMISTRY, Issue 8 2009Teobald Kupka Abstract Efficient B3LYP and BHandH density functionals were used to estimate methanol's nuclear magnetic isotropic shieldings and spin,spin coupling constants in the basis set limit. Polarization-consistent pcS- n and pcJ- n (n = 0, 1, 2, 3 and 4), and segmented contracted XZP, where X = D, T, Q and 5, basis sets were used and the results fitted with simple mathematical formulas. The performance of the methods was assessed from comparison with experiment and higher level calculations. 1J(CH) and 3J(HH) values were determined from very diluted solutions in deuterochloroform and compared with theoretical predictions. The agreement between complete basis set (CBS) density functional theory (DFT) predicted isotropic shieldings and spin,spin values and experiment was good. The BHandH/pcS- n methanol shieldings obtained using structures optimized at the same level of theory are approaching the accuracy of the advanced coupled-cluster-singles-doubles-approximate triples (CCSD(T)) calculations. Copyright © 2009 John Wiley & Sons, Ltd. [source] Modulation of Stacking Interactions by Transition-Metal Coordination: Ab Initio Benchmark StudiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2010Shaun Abstract A series of ab initio calculations are used to determine the CH,,,, and ,,,,,-stacking interactions of aromatic rings coordinated to transition-metal centres. Two model complexes have been employed, namely, ferrocene and chromium benzene tricarbonyl. Benchmark data obtained from extrapolation of MP2 energies to the basis set limit, coupled with CCSD(T) correction, indicate that coordinated aromatic rings are slightly weaker hydrogen-bond acceptors but are significantly stronger hydrogen-bond donors than uncomplexed rings. It is found that ,,,,, stacking to a second benzene is stronger than in the free benzene dimer, especially in the chromium case. This is assigned, by use of energy partitioning in the local correlation method, to dispersion interactions between metal d and benzene , orbitals. The benchmark data is also used to test the performance of more efficient theoretical methods, indicating that spin-component scaling of MP2 energies performs well in all cases, whereas various density functionals describe some complexes well, but others with errors of more than 1,kcal,mol,1. [source] | ||||||||||