Search results for "Basis set"
showing 10 items of 193 documents
Determination of the Relative Stereochemistry of Flexible Organic Compounds by Ab Initio Methods: Conformational Analysis and Boltzmann-Averaged GIAO…
2002
Ab initio calculations at the Hartree-Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) 1 3 C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an openchain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimental 1 3 C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8-13 relevant conformers per stereoisomer. Thermochemical calculations …
Anharmonic vibrational frequency calculations for solvated molecules in the B3LYP Kohn–Sham basis set limit
2012
Abstract The solvent dependence of harmonic and anharmonic vibrational wavenumbers of water, formaldehyde and formamide was studied using the B3LYP method. The results obtained with the hierarchy of Jensen's polarization-consistent basis sets were fitted with two-parameter formula toward the B3LYP Kohn–Sham complete basis set (CBS) limit. Anharmonic corrections have been obtained by a second order perturbation treatment (VPT2) and vibrational configuration interaction (VCI) method. The solvent environment was treated according to the self-consistent reaction field polarizable continuum model (SCRF PCM) approach.
Ab initio calculations and vibrational spectroscopy on the phenylenediamine isomers
1998
Molecular orbital calculations at HF and MP2 levels have been performed using the 6-3IG** basis set for full geometry optimization of the phenylenediamine isomers. Our results show that only a transoid conformer is found for o-phenylenediamine, whereas cis and trans conformers exist for m- and p-phenylenediamine. Vibrational normal modes have been also analyzed for the gas phase and in chloroform solution, and compared with experimental data we have obtained using FTIR spectroscopy. © 1998 Elsevier Science B.V.
Pairing energy effects in cyanide complexes of CpCrIII
1999
International audience; The steric and electronic factors responsible for the reactivity differences between CN and Cl complexes of CpCrIII were examined by DFT/B3LYP computational techniques. The energy difference between quadruplet and doublet [CpCr(CN)2(PH3)], ΔED–Q, was calculated to be 21.2 kcal mol−1 with the LanL2DZ basis set. Although the high-spin configuration is still the ground state for the cyanide complex, the energy gap is 8.7 kcal mol−1 less than that found for the corresponding chloride species. The difference between quadruplet [CpCr(CN)2(PH3)] and doublet [CpCr(CN)2(PH3)2] less free PH3, ΔECr–P, is also smaller than for the Cl system. The components of ΔECr–P for CN and C…
Analytic high-order Douglas–Kroll–Hess electric field gradients
2007
In this work we present a comprehensive study of analytical electric field gradients in hydrogen halides calculated within the high-order Douglas-Kroll-Hess (DKH) scalar-relativistic approach taking picture-change effects analytically into account. We demonstrate the technical feasibility and reliability of a high-order DKH unitary transformation for the property integrals. The convergence behavior of the DKH property expansion is discussed close to the basis set limit and conditions ensuring picture-change-corrected results are determined. Numerical results are presented, which show that the DKH property expansion converges rapidly toward the reference values provided by four-component met…
Basis set and correlation effects in the calculation of accurate gas phase dimerization energies of two M+2 to give M2+4 (M = S, Se)
2000
The dimerization energies of two M2+ to give M42+ (M = S, Se) were calcd. They depend strongly on the size of the basis set and the correlation method used (ranging from 217 to 522 kJ/mol, M = S) and, therefore, a systematic study of basis set and correlation effects was performed [MP2, MP3, MP4(SDQ), CCSD, CCSD(T)]. The introduction of a second set of polarizing d-functions caused a significant redn. of the dimerization energies, but neither of the above limits is reached by the MPn (n = 2, 3, 4) theory, even with the largest basis sets [cc-pVQZ]. However, convergence was achieved by CCSD(T), compd. methods or hybrid HF/DFT calcns. employing flexible basis sets [e.g., CCSD(T)/cc-pV5Z, CBS-…
Vertical spectrum of the C2H 2+ system. An open shell (SC)2-CAS-SDCI study
2003
The open shell (SC)(2)-CAS-SDCI method along with a basis set of atomic natural orbitals (ANO) has been applied for calculating the main ionization potentials of acetylene, as well as the manifold of excited states of the different symmetries up to 32 eV. In this method, the single and double excitations of a CAS space are generated and the corresponding CI matrix is corrected by means of the (SC)(2) procedure that cancels the size-extensivity error and adds some high order contributions. The mean absolute error for the outer-valence X (2)Pi(u)(1pi(u) (-1)), A (2)Sigma(g) (+)(3sigma(g) (-1)), and B (2)Sigma(u) (+)(2sigma(u) (-1)) states, and the inner-valence C (2)Sigma(g) (+)(2sigma(g) (-1…
CCSD-CTOCD static dipole shielding polarizability for quantification of the chiral NMR effects in oxaziridine derivatives
2014
Chiral discrimination by nuclear magnetic resonance (NMR) spectroscopy might be achieved through the pseudo-scalar derived from the dipole shielding polarizability tensor. Coupled Cluster Singles and Doubles-Quadratic Response (CCSD-QR) calculations inside the continuous translation of the origin of the current density formalism have been carried out to determine the effects of basis set, electron correlation, and gauge translation on the determination of this magnitude in oxaziridine derivatives. Inclusion of electronic correlation is needed for adequately describing the pseudo-scalar for the heavier nuclei, making CCSD a rigorous and affordable method to compute these high order propertie…
The molecular structure of N-hydroxyurea
1996
Ab initio calculations were performed on the tautomers and conformers of N-hydroxyurea using a 6-31G** basis set. The minimum-energy structures have been found and the importance of the intramolecular hydrogen bond as the stabilizing factor was pointed out. © 1996 John Wiley & Sons, Inc.
The Ground State Electronic Energy of Benzene.
2020
We report on the findings of a blind challenge devoted to determining the frozen-core, full configuration interaction (FCI) ground state energy of the benzene molecule in a standard correlation-consistent basis set of double-$\zeta$ quality. As a broad international endeavour, our suite of wave function-based correlation methods collectively represents a diverse view of the high-accuracy repertoire offered by modern electronic structure theory. In our assessment, the evaluated high-level methods are all found to qualitatively agree on a final correlation energy, with most methods yielding an estimate of the FCI value around $-863$ m$E_{\text{H}}$. However, we find the root-mean-square devia…