Search results for "Basis set"
showing 10 items of 193 documents
GIAO/DFT 13C NMR Chemical Shifts of 1,3,4-Thiadiazoles
2007
1 H, 13 C and 15 N NMR spectra of 2-acetylamino-1,3,4-thiadiazole and its 5-substituted derivatives have been measured and assigned based on reference data, as well as homo- and heteronuclear 2 D NMR experiments. In addition, the GIAO/DFT approach at the B3LYP level of theory using the 6-311G basis set was used to calculate the 13 C NMR chemical shifts. Although this method gives reliable results for 2-arylhydrazones of 1,3-diphenylpropanetrione, 2-phenacylpyridines, (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines, 4-fluoroanilines, (1Z,3Z)-1,4-di(pyridin-2-yl)buta-1,3-dienediols and their tautomeric forms, the calculated chemical shifts for the 1,3,4-thiadiazoles studied are less satisfactory. Pr…
X‐ray Absorption Spectra of Cu II and Cu III Complexes of N , N ′ ‐1,2‐Phenylenebis(2‐mercapto‐2‐methylpropionamide)
2005
The X-ray absorption spectra of Cu I I and Cult complexes of N,N'-1,2-phenylenebis(2-mercapto-2-methylpropionamide) were recorded and analysed in solid phase. The EXAFS spectra of the two samples were refined with full multiple scattering path. Geometry optimisations on the Cu I I and Cu I I I complexes were performed by the B3LYP density functional method, with the 6-31G(d,p) basis set, considering different spin multiplicities. The singlet state of the Cu I I I complex was shown to be more stable than the triplet state, and a good agreement between the calculated and the corresponding experimental structure was found. Further single-point calculations on the optimised geometry were carrie…
Theoretical rotational constants of MeCnN species
1990
Abstract By means of SCF HF “ab initio” calculations with STO-3G and 6-31G basis sets, the geometric parameters of methylcyanopolyynes (MeCnN n=3, 5, 7 and 9) have been obtained. B0=0.3748 GHz for MeC7N and B0=0.2708 GHz for MeC9N, with a STO-3G basis set, were obtained. Both species, unstable in the laboratory, are still undetected in the interstellar medium, although their existence is very probable.
Predicting the structure and vibrational frequencies of ethylene using harmonic and anharmonic approaches at the Kohn–Sham complete basis set limit
2016
In this work, regular convergence patterns of the structural, harmonic, and VPT2-calculated anharmonic vibrational parameters of ethylene towards the Kohn–Sham complete basis set (KS CBS) limit are demonstrated for the first time. The performance of the VPT2 scheme implemented using density functional theory (DFT-BLYP and DFT-B3LYP) in combination with two Pople basis sets (6-311++G** and 6-311++G(3df,2pd)), the polarization-consistent basis sets pc-n, aug-pc-n, and pcseg-n (n = 0, 1, 2, 3, 4), and the correlation-consistent basis sets cc-pVXZ and aug-cc-pVXZ (X = D, T, Q, 5, 6) was tested. The BLYP-calculated harmonic frequencies were found to be markedly closer than the B3LYP-calculated h…
Molecular structures of Se(SCH3)2 and Te(SCH3)2 using gas-phase electron diffraction and ab initio and DFT geometry optimisations
2005
The molecular structures of Se(SCH3)2 and Te(SCH3)2 were investigated using gas-phase electron diffraction (GED) and ab initio and DFT geometry optimisations. While parameters involving H atoms were refined using flexible restraints according to the SARACEN method, parameters that depended only on heavy atoms could be refined without restraints. The GED-determined geometric parameters (rh1) are: rSe-S 219.1(1), rS-C 183.2(1), rC-H 109.6(4) pm; S-Se-S 102.9(3), Se-S-C 100.6(2), S-C-H (mean) 107.4(5), S-Se-S-C 87.9(20), Se-S-C-H 178.8(19)° for Se(SCH3)2, and rTe-S 238.1(2), rS-C 184.1(3), rC-H 110.0(6) pm; S-Te-S 98.9(6), Te-S-C 99.7(4), S-C-H (mean) 109.2(9), S-Te-S-C 73.0(48), Te-S-C-H 180.…
Complete basis set B3LYP NMR calculations of CDCl3solvent's water fine spectral details
2008
The assignment of singlet at 1.55 ppm and the 1:1:1 triplet at 1.519 ppm to H2O and HOD in the 400 MHz 1H NMR spectrum of CDCl3 solvent were supported by complete basis set (CBS) GIAO-B3LYP calculated chemical shift and the CBS B3LYP estimated 2J(D,H) spin–spin coupling constant (SSCC). The CBS fitting of B3LYP/cc-pCVxZ and B3LYP/pcJ-n predicted SSCC values, the accurate value of 2J(D,H) = − 1.082 ± 0.030 Hz of HOD in chloroform-d1 and the H/D isotopic shift of 0.0307(1) ppm were reported for the first time. The agreement between CBS B3LYP predicted chemical shift, spin–spin values and experiment was good. Copyright © 2008 John Wiley & Sons, Ltd.
Oxygen and vacancy defects in silicon. A quantum mechanical characterization through the IR and Raman spectra.
2021
The Infrared (IR) and Raman spectra of various defects in silicon, containing both oxygen atoms (in the interstitial position, Oi) and a vacancy, are computed at the quantum mechanical level by using a periodic supercell approach based on a hybrid functional (B3LYP), an all-electron Gaussian-type basis set, and the Crystal code. The first of these defects is VO: the oxygen atom, twofold coordinated, saturates the unpaired electrons of two of the four carbon atoms on first neighbors of the vacancy. The two remaining unpaired electrons on the first neighbors of the vacancy can combine to give a triplet (Sz = 1) or a singlet (Sz = 0) state; both states are investigated for the neutral form of …
Interstitial carbon defects in silicon. A quantum mechanical characterization through the infrared and Raman spectra.
2021
The infrared (IR) and Raman spectra of eight substitutional carbon defects in silicon are computed at the quantum mechanical level by using a periodic supercell approach based on hybrid functionals, an all electron Gaussian type basis set and the CRYSTAL code. The single substitutional C s case and its combination with a vacancy (C s V and C s SiV) are considered first. The progressive saturation of the four bonds of a Si atom with C is then examined. The last set of defects consists of a chain of adjacent carbon atoms C s i , with i = 1-3. The simple substitutional case, C s , is the common first member of the three sets. All these defects show important, very characteristic features in th…
Periodic models in quantum chemical simulations ofF centers in crystalline metal oxides
2007
We present a survey of recent first principles simulations of the neutral oxygen vacancies (F centers) existing as native or radiation-induced point defects in various crystalline metal oxides in different forms (bulk, bare substrate surface, and on the interface with metal adsorbates). We mainly consider periodic models in calculations of point defects using the metal oxide supercell or cyclic clusters. We compare different formalisms of first principles calculations, mostly the Density Functional Theory (DFT) as implemented in the framework of either localized basis set of atomic orbitals or delocalized basis sets of plane waves. We analyze in detail the structural and electronic properti…
Electronic structure calculations with GPAW: a real-space implementation of the projector augmented-wave method.
2011
Electronic structure calculations have become an indispensable tool in many areas of materials science and quantum chemistry. Even though the Kohn-Sham formulation of the density-functional theory (DFT) simplifies the many-body problem significantly, one is still confronted with several numerical challenges. In this article we present the projector augmented-wave (PAW) method as implemented in the GPAW program package (https://wiki.fysik.dtu.dk/gpaw) using a uniform real-space grid representation of the electronic wavefunctions. Compared to more traditional plane wave or localized basis set approaches, real-space grids offer several advantages, most notably good computational scalability an…