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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 8182-8187 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reduced frozen-core approximation (RFCA) that has been previously used for nonrelativistic calculations is extended to relativistic calculations. The RFCA adopts a new method for the orthogonalization of valence basis functions to core orbitals. Orthogonalization is performed using corelike basis functions consisting of fewer primitive basis functions than core orbitals. Dirac–Fock–Roothaan calculations on HI and ThO show that the relativistic RFCA can reduce computing time and closely reproduce the total and valence orbital energies and spectroscopic constants obtained by all-electron calculations. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3738-3739 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate all-electron Dirac–Fock–Roothaan calculations were performed for the 1Σ+ ground state of ThO using Gaussian basis sets of Th (24s20p16d12f)/[19s15p12d5f] and O (10s6p1d)/[7s4p1d]. The relativistic effects in ThO were found to slightly extend the bond length re, to increase the dissociation energy De, and to decrease the harmonic frequency ωe. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5179-5184 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It has been found that independent parameters in the variation of a one-electron density matrix (DM) for closed-shell systems are elements of its unitary transformed matrix and, in a special case, reduce to the rotation parameters that connect the occupied and virtual orbital spaces in the exponential transformed self-consistent field method. To obtain the unitary matrix of transformation, a simpler method of orthogonalizing the column vectors of the DM has been proposed instead of its diagonalization. An iterative method has been formulated to determine these independent parameters. Several test calculations using this method reproduced the results using the Hartree–Fock–Roothaan method. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 1063-1067 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Compact formulas are derived of molecular energy integrals over real solid spherical Gaussian-type functions using the Talmi transformation. A new method for calculating the Talmi coefficients is presented, which does not require angular-momentum algebra of 3j and 9j symbols. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 2271-2275 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: All-electron relativistic and nonrelativistic self-consistent-field programs have been developed and calculations on diatomics HI, HI+, AtH, and AtH+ have been performed to get the spectroscopic constants. The results of HI, HI+, and AtH agree fairly well with those by the effective-core potential methods. The relativistic contractions of bond lengths of AtH and AtH+ are found to be about 2%. Contrary to the common belief, the harmonic frequencies found from the relativistic calculations are smaller than those of the nonrelativistic calculations.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 6773-6777 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A frozen-core approximation (FCA) is proposed using a new orthogonalization method of the valence basis functions to the core orbitals. The orthogonalization is done through corelike basis functions in contrast to the usual Schmidt process in which the valence basis functions are directly orthogonalized to the core orbitals. The FCA using the new orthogonalization is mathematically rigorous and attains saving of the computation using the reduced corelike basis functions. Numerical calculations on diatomic molecules HCl, N2, P2, and Cl2 reproduced well the results of all-electron calculations. Discussion on the proposed FCA is given especially in connection with the model-potential methods of Huzinaga and co-workers.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5013-5019 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A systematic calculation of new relativistic Gaussian basis sets is reported. The new basis sets are similar to the previously reported ones [J. Chem. Phys. 91, 4193 (1989)], but, in the calculation, the Breit interaction has been explicitly included besides the Dirac–Coulomb Hamiltonian. They have been adopted for the calculation of the self-consistent field effect on the Breit interaction energies and are expected to be useful for the studies on higher-order effects such as the electron correlations and other quantum electrodynamical effects.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4364-4371 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Fieck's formulation of molecular integrals over the spherical Laguerre Gaussian-type functions, L(l+1/2)n(αr2) exp(−αr2)rlYlm(θ,φ) [where L(s)n(x) is the Sonine (or the generalized Laguerre) polynomial and Ylm(θ,φ) is the complex spherical harmonic], has been generalized to include four types of operators: Ynlm(r), Yˆnlm(∇), Yˆnlm(∇)(1/r), and Yˆnlm(∇12)(1/r12), where Ynlm(r) is the homogeneous solid harmonic and Yˆnlm(∇) its operator. These operators represent such various kinds of molecular properties as the energy (i.e., the overlap, the kinetic energy, the nuclear attraction, and the electron repulsion), the electric multipole moments and transitions, the electric field and gradient, the quasirelativistic interactions (e.g., the mass velocity, the spin–orbit, and the spin–spin), etc. Attention has been paid on the aspects of numerical computation of their molecular integrals so that the integral formulas are suitable for computation in the scheme of "shell structure'' and over the contracted Gaussian-type functions.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4193-4196 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Exponent parameters of the nonrelativistically optimized well-tempered Gaussian basis sets of Huzinaga and Klobukowski have been employed for Dirac–Fock–Roothaan calculations without their reoptimization. For light atoms He (atomic number Z=2)–Rh (Z=45), the number of exponent parameters used has been the same as the nonrelativistic basis sets and for heavier atoms Pd (Z=46)–Hg(Z=80), two 2p (and three 3d) Gaussian basis functions have been augmented. The scheme of kinetic energy balance and the uniformly charged sphere model of atomic nuclei have been adopted. The qualities of the calculated basis sets are close to the Dirac–Fock limit.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 39 (1975), S. 111-118 
    ISSN: 1432-2234
    Keywords: Random-phase approximation ; Triplet instability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Random-phase approximations (RPA) have been applied to the calculation of the triplet π-π* transition spectra of 18 conjugated molecules in the framework of Pariser-Parr-Pople approximations. It is found that the normal RPA (n-RPA) shows the triplet instability for most molecules in the Nishimoto-Mataga approximation of electron-repulsion integrals. However, it is shown that this instability can be circumvented by the use of the renormalized RPA (r-RPA) in which the correlated ground states are calculated by the second-order perturbation theory. It is also shown that even in the n-RPA the suitable parametrization of electron-repulsion integrals removes this instability. It is ascertained that such an increasing order of energies as ω(n-RPA)〈ω(Tamm-Dancoff approximation)〈ω(r-RPA) holds for most of energy levels.
    Type of Medium: Electronic Resource
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