1,721,096 research outputs found
Random-phase calculations of frequency-dependent polarizabilities and hyperpolarizabilities of long polyene chains
No full textWe apply the double-direct random-phase-approximation (DDRPA) method [Gren et al., J. Chem. Phys. 98, 6417 (1993)] to calculate the static and dynamic polarizabilities and hyperpolarizabilities for a sequence of polyene chains. Like the direct self-consistent-field method, DDRPA is driven directly by the atomic-orbital integrals. It further uses iterative techniques based on direct linear transformations for solving the RPA eigenvalue equations and sets of linear equations. This allows applications to long chains, including C28H30 in the present study. The calculated optical spectra, viz., excitation energies and transition moments, the polarizabilities, and the hyperpolarizabilities are in excellent agreement with available experimental data. Computations on the longer polyenes are facilitated by the weak basis-set dependency on chain length for the longitudinal components of the (hyper)polarizabilities. The length dependence of the dispersion is significant even at small frequencies, and is quite different for the various tensorial components and for the averaged static, Kerr, and electrical-field-induced second harmonic generation values. Some of the results can be rationalized by the observation that the most intensive 1Bu transition also determines the band gap, and that the band gap converges very slowly with respect to chain length. The correlation length of the static polarizability is predicted to be about 40 unit cells, while for the static hyperpolarizability the prediction is well over 100. © 1995 The American Physical Society
The molecular structure of ferrocene
No full textThe vertical iron-cyclopentadienyl distance in ferrocene has been determined using ab initio coupled-cluster theory. At the CCSD level, a TZV2P basis set yields a metal-ligand distance of 1.664 à with all 96 electrons correlated and 1.672 à with 66 electrons correlated. With 66 electrons correlated at the CCSD(T) level the equilibrium distance is 1.660 Ã, in agreement with experiment (1.66 Ã). © 1996 American Institute of Physics
A direct atomic orbital driven implementation of the coupled cluster singles and doubles (CCSD) model
No full textThe coupled cluster singles and doubles (CCSD) model has been implemented using a direct atomic integral driven technique. The atomic integrals are generated in distributions with one fixed and three free indices, and one distribution is stored in fast memory together with the cluster amplitudes and the cluster vector function. Little loss in efficiency has been obtained compared to a molecular orbital integral driven technique. Sample calculations are presented for HFCO containing 328 basis functions. © 1994
Integral direct calculation of CC2 excitation energies: singlet excited states of benzene
No full textBasis-set convergence of correlated calculations on water
No full textThe basis-set convergence of the electronic correlation energy in the water molecule is investigated at the second-order Mà ̧ller-Plesset level and at the coupled-cluster singles-and-doubles level with and without perturbative triples corrections applied. The basis-set limits of the correlation energy are established to within 2 mEhby means of (1) extrapolations from sequences of calculations using correlation-consistent basis sets and (2) from explicitly correlated calculations employing terms linear in the interelectronic distances rij. For the extrapolations to the basis-set limit of the correlation energies, fits of the form a + bX-3(where X is two for double-zeta sets, three for triple-zeta sets, etc.) are found to be useful. CCSD(T) calculations involving as many as 492 atomic orbitals are reported. © 1997 American Institute of Physics
First-order one-electron properties in the integral-direct coupled cluster singles and doubles model
No full textAn integral-direct implementation of first-order one-electron properties in the coupled cluster singles and doubles (CCSD) model is presented. The implementation increases the range of applicability of CCSD first-order one-electron property calculations significantly compared to nondirect approaches. As an application a thorough basis set investigation is performed on five diatomic molecules at the Hartree-Fock and CCSD levels for the molecular electric dipole moment, the molecular electric quadrupole moment, and the electric field gradient at the nuclei. In general, basis sets of polarized triple-zeta quality are the smallest to be recommended, and the convergence towards the basis set limit is faster at the Hartree-Fock level than at the CCSD level. Among the properties considered, the electric dipole moment is the easiest to converge. The electric dipole and especially the electric quadrupole moment require diffuse functions for high accuracy. With standard basis sets, it is not possible to calculate electric field gradients consistently within three thousandths of an atomic unit of the basis set limit - for this purpose, elaborate nonstandard basis sets are required. The electric field gradients at the nuclei in HCN and the electric dipole moment of the furan molecule are calculated at the CCSD level employing up to 417 basis functions, further demonstrating the large-scale applicability of the implementation. © 1997 American Institute of Physics
A systematic ab initio study of the water dimer in hierarchies of basis sets and correlation models
No full text
Research note: Scf calculations of the nmr shielding tensor for the ethylenic carbon atom in C3Cl4
No full textLarge differences have been reported in previous SCF calculations of the NMR shielding tensor of the ethylenic carbon atom in tetrachlorocyclopropene. Our calculations show that these differences are due to the use of different molecular geometries, not to differences in the methods applied to obtain gauge-invariance. © 1995 Taylor & Francis Ltd
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