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  • American Institute of Physics (AIP)  (6)
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  • 1
    Electronic Resource
    Electronic Resource
    Semiclassical calculation of the vibrational structure of the B˜ 1Bu Rydberg state of trans-di-imide from ab initio configuration interaction potential energy surfaces (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3027-3035 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The applicability of the SDCI and CEPA methods to the excited state of the title molecule is investigated. Two basis sets are used, one of triple zeta quality extended with diffuse functions and another which also contained polarization functions. For the subsequent CI calculations closed shell as well as open shell, molecular orbitals are used. We investigate the Pople correction as a way to obtain size consistent results from the SDCI calculation. For each method, a two-dimensional potential energy surface of the 1(4ag,5bu) Rydberg state of trans-di-imide (HNNH) is calculated. The vibrational fine structure in the corresponding B˜←X˜ UV absorption spectrum is derived from these surfaces and the result is compared to the spectrum measured by Neudorfl et al. [P. S. Neudorfl, R. A. Back, and A. E. Douglas, Can. J. Chem. 59, 506 (1981)]. A semiclassical method [K. S. Sorbie, Mol. Phys. 32, 1577 (1976)] is used to obtain the vibrational frequencies. A slightly modified version of the Heller frozen Gaussian approximation [E. J. Heller, J. Chem. Phys. 75, 2923 (1981)] is proposed and used to obtain the intensities of the vibrational bands. We conclude that it is important to use the open shell molecular orbital basis and the SDCI plus Pople correction, or even better, the CEPA. Both methods give good results for the vertical transition energy and excited state geometry. The error in the vibrational frequencies is in the order of 10%, but the NN-stretch mode is best described by the CEPA method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456924
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  • 2
    Electronic Resource
    Electronic Resource
    Gradients in valence bond theory (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 2100-2108 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A gradient method for general valence bond wave functions is presented. The electronic energy is used as a Lagrange multiplier. The derivatives of the normalization and of the first- and second-order cofactors present in the energy expression have to be evaluated, giving rise to first-, second-, and third-order cofactors. This evaluation is done using an extension of methods described previously. The use of gradients is illustrated with some calculations on organic molecules, viz. ethene, 1, 4-butadiene, and benzene. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.482021
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  • 3
    Electronic Resource
    Electronic Resource
    An improved ab initio relativistic zeroth-order regular approximation correct to order 1/c2 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9957-9965 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The equations of the original ab initio scalar-relativistic zeroth-order regular approximation (ZORA) and the infinite-order regular approximation (IORA) are expanded in orders of 1/c2. It is shown that previous ZORA/IORA implementations in ab initio quantum chemistry programs were not correct to order 1/c2, but contained imperfections leading to fictitious self-interactions. These errors can be avoided by adding exchange-type terms (coupling the large and small components) to the relativistic ZORA correction to the Hamiltonian, yielding improved ab initio relativistic zeroth- and infinite-order regular approximations that are correct to order 1/c2. The new methods have been tested numerically by computing the total energies, orbital energies, and static electric dipole polarizabilities of the rare gas atoms He through Xe. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1323266
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  • 4
    Electronic Resource
    Electronic Resource
    Benchmark full configuration interaction calculations on the helium dimer (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 7479-7483 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Full configuration interaction calculations are presented for the helium dimer employing large basis sets. Using the best basis, which contains up to h-type basis functions and several closely spaced sets of bond functions, the interaction energy was calculated for a variety of internuclear distances in the range 4.0 to 12.0 bohr. The best calculated values for the He2 interaction energy are −10.947 K at 5.6 bohr (the van der Waals minimum) and +294.90 K at 4.0 bohr (on the repulsive wall). The interaction energy at 4.0 bohr differs significantly from the most recent semiempirical potential of Aziz and Slaman [J. Chem. Phys. 94, 8047 (1991)], indicating that this potential is too attractive around 4.0 bohr. Using a more generally accessible basis, containing only up to f-type basis functions and only one set of bond functions, the interaction energy was calculated to be −10.903 K at 5.6 bohr and +294.96 K at 4.0 bohr. These results show that functions of higher than f symmetry and bond functions distributed over several centers are necessary for obtaining highly accurate results, particularly at the van der Waals minimum. Our results may be used to benchmark more approximate methods. The CCSD(T) method is estimated to underestimate the full CI interaction energy by 0.33 K at 5.6 bohr and by 2.0 K at 4.0 bohr. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469060
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  • 5
    Electronic Resource
    Electronic Resource
    Compact valence bond functions with breathing orbitals: Application to the bond dissociation energies of F2 and FH (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5969-5976 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An original computational method of ab initio valence bond type is proposed, aiming at yielding accurate dissociation energy curves, while dealing with wave functions being very compact and clearly interpretable in terms of Lewis structures. The basic principle is that the wave function is allowed to have different orbitals for different valence bond structures. Thus, throughout the dissociation process, the so-called "breathing orbitals'' follow the instantaneous charge fluctuations of the bond being broken by undergoing changes in size, hybridization, and polarization. The method is applied to the dissociation of F2 and FH. For each molecule, a wave function involving only three valence bond configurations yields equilibrium bond lengths within 0.01 A(ring), and dissociation energies within about 2 kcal/mol of the results of estimated or true full configuration interaction in the same basis sets. The effect of dynamical electron correlation on calculated dissociation energies is analyzed. It is shown that restricting the correlation to its nondynamical part results in an improper treatment of ionic terms due to a mean-field compromise in the optimization of the orbitals.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468459
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  • 6
    Electronic Resource
    Electronic Resource
    Accuracy of the Boys and Bernardi function counterpoise method (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 4728-4737 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The performance of the Boys and Bernardi function counterpoise (FCP) method in eliminating the basis set superposition error (BSSE) is studied for He2, at R=5.6 a.u., within the supermolecular coupled electron pair approximation (CEPA-1) method. A series of one-electron Gaussian basis sets is designed that allows a systematic approach to the basis set limit value of the interaction energy. Every basis set contains a part suitable to reproduce the atomic correlation energy and a second part optimized for the dispersion interaction in He2. BSSE-free correlated first-order interaction energies [E(1)], calculated using perturbation theory, are reported for each of these sets. Extrapolation to the basis set limit yields a new value of 33.60±0.02 μH for E(1) at R=5.6 a.u. Extending previous work, the supermolecular CEPA-1 interaction energies for each set are then compared to the total of E(1) and the BSSE-free Møller–Plesset second-order dispersion energy reported previously. While for some basis sets the uncorrected ΔE values deviate up to 43 K from the perturbation estimate, the FCP-corrected results always agree within 0.4 K. A virtuals-only counterpoise procedure is considered as well, but fails badly. The remaining discrepancies in the FCP results are ascribed to a failure of the Møller–Plesset approach to precisely model the dispersion energy at the CEPA level. This problem is removed in a further, more stringent test where supermolecular EintCEPA-intra results, in which only the intra-atomic correlation (at the CEPA-1 level) is taken into account, are directly compared to the BSSE-free E(1) values. In this test the FCP-corrected supermolecular results agree, for the larger sets, to within 0.001 K with the results expected on the basis of E(1). These findings demonstrate, for the first time, that at least in He2 the FCP recipe yields interaction energies that correspond precisely (to machine precision) to the basis set and correlation method at hand.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465106
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