1,509 research outputs found
Nauwkeurige ab initio berekening van anharmonische krachtvelden en sceptroscopische constanten van kleine polyatomische moleculen
No full textNauwkeurige ab initio berekening van anharmonische krachtvelden en sceptroscopische constanten van kleine polyatomische moleculen
No full textAb initio study of cluster molecules relevant to materials science and astrophysics: development of combined bond-polarization basis sets for the accurate ab initio calculation of dissocation energies
No full textAb initio study of cluster molecules relevant to materials science and astrophysics: development of combined bond-polarization basis sets for the accurate ab initio calculation of dissocation energies
No full textOn the performance of correlation consistent basis sets for the calculation of total atomization energies, geometries, and harmonic frequencies
No full textThe total atomization energies (Sigma D-e values), geometries, and harmonic frequencies for a number of experimentally well-described molecules have been calculated at the CCSD(T) (coupled cluster) level using Dunning's correlation-consistent cc-pVDZ([3s2p1d]), cc-pVTZ([4s3p2d1f]), and cc-pVQZ([5s4p3d2f1g]) basis sets. Additivity correction are proposed for binding energies and geometries. Using a three-term additive correction of the form proposed by Martin [J. Chem. Phys. 97, 5012 (1992)] mean absolute errors in Sigma D-e are 0.46 kcal/mol for the cc-pVQZ, 0.93 for the cc-pVTZ, and 2.59 for the c-pVDZ basis sets. The latter figure implies that, although unsuitable for quantitatively accurate work, three-term corrected CCSD(T)/cc-pVDZ binding energies can still be used for a rough estimate when the cost of larger basis set calculations would be prohibitive. CCSD(T)/cc-pVQZ calculations reproduce bond lengths to 0.001 Angstrom for single bonds, and 0.003 Angstrom for multiple bonds; remaining error is probably partly due to core-core and core-valence correlation. CCSD(T)/cc-pVTZ calculations result in additional overestimates of 0.001 Angstrom for single, 0.003 Angstrom for double, and 0.004 Angstrom for triple bonds. CCSD(T)/cc-pVDZ calculations result in further overestimates of 0.01 Angstrom for single bonds, and 0.02 Angstrom for multiple bonds. CCSD(T)/cc-pVDZ harmonic frequencies are in surprisingly good agreement with experiment, except for pathological cases like the umbrella mode in NH3. Both CCSD(T)/cc-pVTZ and CCSD(T)/cc-pVQZ harmonic frequencies generally agree with experiment to 10 cm(-1) or better; performance of cc-pVQZ is somewhat superior on multiple bonds or the umbrella mode in NH3. Again, a source of remaining error appears to be core correlation. The use of MP2/6-31G* reference geometries in the Sigma D-e calculation can result in fairly substantial errors in the uncorrected Sigma D-e values fbr systems with cumulated multiple bonds. These errors however appear to be largely absorbed by the three-term correction. Use of CCSD(T)/cc-pVDZ reference geometries appears to have no detrimental effect on computed Sigma D-e values and is recommended for cases where only single-point calculations in the cc-pVTZ basis set are possible
Supporting information for CCSD(F12*) vs. CCSD-F12b vs. CCSD paper
No full textSupporting information for manuscript, "Do CCSD and approximate CCSD-F12 variants converge to the same basis set limits? The case of atomization energies" by Manoj K. Kesharwani, Nitai Sylvetsky, Andreas Köhn, David P. Tew, and Jan M.L. Martin, J. Chem. Phys., accepted with revision
Basis set convergence in second-row compounds: the importance of core polarization functions
No full textUsing sequences of Dunning's correlation consistent basis sets, cc-pVnZ (n = 3,4,5), convergence of molecular properties is much slower for second-row compounds than for their first-row counterparts, both at the Hartree-Fock and at correlated levels, due to core polarization effects. By adding a single high-exponent d function to the standard cc-pVnZ basis sets, convergence is greatly accelerated. After correcting for core correlation, computed D-0, r(e), and omega(e) values for a number of diatomics generally agree with experiment to better than 0.02 eV, 0.001 Angstrom, and 5 cm(-1), respectively. (C) 1998 Elsevier Science B.V
The geometry, vibrational frequencies, and total atomization energy of ethylene: a calibration study
No full textThe anharmonic part of a recently calculated ab initio quartic force field for ethylene has been combined with geometries and harmonic frequencies at higher levels of theory, including expansion to spdfg basis sets and inclusion of core correlation. Resulting fundamentals and ground-state rotational constants have been compared with experiment, Our best estimate for the r(e) geometry is r(e)(CC) = 1.3307(3) Angstrom, r(e)(CH) = 1.0809(3) Angstrom, theta(e)(CCH) = 121.44(3)degrees, which reproduces the experimental rotational constants to 0.01%. The experimental fundamentals and main resonance partners are calculated with a mean absolute error of 2.3 cm(-1). Our best calculated total atomization energy, 531.7(5) kcal/mol, falls within the error bar of the experimental value 531.9(3) kcal/mol
Density-functional theory concepts and techniques for studying molecular charge distributions and related properties
No full textOn the integration accuracy in molecular density functional theory calculations using Gaussian basis sets
No full textThe sensitivity of computed DFT (Density Functional Theory) molecular properties (including energetics, geometries, vibrational frequencies, and infrared intensities) to the radial and angular numerical integration grid meshes, as well as to the partitioning scheme, is discussed for a number of molecules using the Gaussian 98 program system. Problems with typical production grid sizes are particularly acute for third-row transition metal systems, but may still result in qualitatively incorrect results for a molecule as simple as CCH. Practical recommendations are made with respect to grid choices for the energy (+ gradient) steps, as well as for the solution of the CPKS (Coupled Perturbed Kohn-Sham) equations. (C) 2001 Elsevier Science B.V. All rights reserved
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