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  • 11
    Electronic Resource
    Electronic Resource
    Electron correlation, basis sets, and the methylene singlet–triplet gap (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 862-865 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of basis set and electron correlation on the singlet–triplet splitting (ΔEST) of CH2 is examined using the generalized valence bond (GVB) approach. For a standard double zeta plus polarization basis, the GVB based calculation (with only 20–25 spin eigenfunctions) approaches the full CI result (∼220 000 spin eigenfunctions) of Bauschlicher and Taylor to within 0.5 kcal/mol for this basis, but both differ substantially from experiment (errors of 2.4 and 2.9 kcal/mol for GVB and full CI, respectively). We have studied the convergence of ΔEST with basis set and find that an extremely extended basis (triple zeta sp, diffuse sp, triple zeta d, double zeta f ) for GVB yields ΔEST=9.03 kcal/mol, in excellent agreement with the experimental value of 9.09±0.20 kcal/mol.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452287
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  • 12
    Electronic Resource
    Electronic Resource
    Periodic density functional embedding theory for complete active space self-consistent field and configuration interaction calculations: Ground and excited states (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 42-54 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We extend our recently reported embedding theory [J. Chem. Phys. 110, 7677 (1999)] to calculate not only improved descriptions of ground states, but now also localized excited states in a periodically infinite condensed phase. A local region of the solid is represented by a small cluster for which high quality quantum chemical calculations are performed. The interaction of the cluster with the extended condensed phase is taken into account by an effective embedding potential. This potential is calculated by periodic density functional theory (DFT) and is used as a one-electron operator in subsequent cluster calculations. Among a variety of benchmark calculations, we investigate a CO molecule adsorbed on a Pd(111) surface. By performing complete active space self-consistent field, configuration interaction (CI), and Møller–Plesset perturbation theory of order n (MP-n), we not only were able to obtain accurate adsorption energies via local corrections to DFT, but also vertical excitation energies for an internal (5σ→2π*) excitation within the adsorbed CO molecule. We demonstrate that our new scheme is an efficient and accurate approach for the calculation of local excited states in bulk metals and on metal surfaces. Additionally, a systematic means of improving locally on ground state properties is provided. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1420748
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  • 13
    Electronic Resource
    Electronic Resource
    Local weak-pairs pseudospectral multireference configuration interaction (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 1982-1993 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a new reduced scaling multireference singles and doubles configuration interaction (MRSDCI) algorithm based upon the combination of local correlation and pseudospectral methods. Taking advantage of the locality of the Coulomb potential, the weak-pairs approximation of Saebø and Pulay is employed to eliminate configurations having simultaneous excitations out of pairs of distant, weakly interacting orbitals. In conjunction with this, the pseudospectral approximation is used to break down the most time-consuming two-electron integrals into a product of intermediate quantities depending on no more than two orbital indices. The resulting intermediate quantities are then used directly in the CI equations to substantially reduce the number of floating point operations required for diagonalization of the Hamiltonian. Additionally, our CI algorithm is based upon the symmetric group graphical approach CI (SGGA-CI) of Duch and Karwowski. For the purpose of developing reduced scaling CI algorithms, this approach has some important advantages. The most important of these advantages are the on-the-fly calculation of integral coupling coefficients and the separation of the spin and spatial parts of the wave function, which simplifies implementation of local correlation approximations. We apply the method to determine a series of binding energies in hydrocarbons and show that the approximate method predicts binding energies that are within a few kcal/mol of those predicted by the analytic nonlocal method. For large molecules, the local pseudospectral method was shown to be over 7 times as fast as the analytic nonlocal method. We also carry out a systematic study on the performance of different basis sets in the weak-pairs method. It was determined that triple-ζ basis sets were capable of recovering only 99.0% of the correlation energy, whereas double-ζ basis sets recovered 99.9% of the correlation energy. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1487816
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  • 14
    Electronic Resource
    Electronic Resource
    O(N3) scaling of two-electron integrals during molecular geometry optimization (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 6562-6569 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We propose an optimization technique designed specifically for molecular structure optimization performed on an ab initio level. This gradient-based technique is a modification of quadratically convergent quasi-Newton method, and although it requires more energy evaluations than the conventional method, each of these energy evaluations is much cheaper due to O(N3) scaling of the two-electron integrals evaluation. Statistics obtained from numerous optimization runs with Lennard-Jones molecules shows that the number of energy and gradient evaluations for the proposed technique is only 1.5–5 times (for 3–27 atoms, respectively) larger than that for conventional method. Given the great advantage of O(N3) scaling of the two-electron integrals in the former, a substantial speedup of the overall computation can be achieved in certain cases. We consider the factors which affect the performance of the proposed technique and we also present timings and other details of several molecular structure optimization tests of the method on the ab initio level. Additionally, a novel approach to numerical Hessian evaluation during optimization is proposed, where the quality of the Hessian so obtained can be assessed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467065
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  • 15
    Electronic Resource
    Electronic Resource
    Direct inversion in the iterative subspace-induced acceleration of the ridge method for finding transition states (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5437-5441 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We extend the application of the direct inversion in the iterative subspace (DIIS) technique to the ridge method for finding transition states (TS). The latter is not a quasi-Newton-type algorithm, which is the only class of geometry optimization methods that has been combined with DIIS. With this new combination, we obtain a factor of two speedup due to DIIS, similar to the DIIS-related speedup achieved in other methods including quasi-Newton geometry optimization and self-consistent field iterations. We also demonstrate that DIIS is useful even in cases where optimization is started far from the quadratic region of the TS, provided that only one previous iteration is used in the DIIS expansion. We compare the performance of the new ridge-DIIS method to that of the TS algorithm utilized in GAUSSIAN 92. We find that the computational cost of the former is similar (when both methods converge) to that of the latter. The examples considered in the paper include a novel TS found for an isomerization of a cluster of six Na atoms. Locating such a TS poses a known problem for second-derivatives-based algorithms that fail on very flat potential energy surfaces. Thus, the gradient-based ridge-DIIS method is the only TS search method that is robust, does not need second derivatives and/or an initial guess for the TS geometry, and whose performance matches or exceeds that of a second-derivatives-based algorithm. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470579
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  • 16
    Electronic Resource
    Electronic Resource
    Orbital-based direct inversion in the iterative subspace for the generalized valence bond method (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 1251-1256 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an algorithm that is a new combination of the direct inversion in the iterative subspace (DIIS) and the generalized valence bond (GVB) methods. The proposed algorithm is based on applying the DIIS directly to the orbitals updated via the GVB scheme as opposed to the conventional approach of applying DIIS to a series of composite Fock matrices (CFMs). The new method results in GVB convergence in situations where the CFM-based GVB-DIIS cannot be applied at all, e.g., when the original GVB method diverges. When both the new and the conventional methods converge, the former achieves the same reduction in the number of self-consistent field (SCF) iterations as the latter, but using considerably less storage and DIIS-related CPU time. Also, the orbital-based GVB-DIIS is less sensitive to the proximity of an initial guess to the exact wave function, and it does not depend on empirical criteria used in the CFM-based GVB-DIIS. Finally, the orbital-based DIIS formulation is not limited to GVB; it can be easily incorporated into any SCF approach that involves an iterative updating of the orbitals, such as, e.g., multiconfiguration SCF or Kohn–Sham density-functional theory. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468912
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  • 17
    Electronic Resource
    Electronic Resource
    First-principles-derived dynamics of F2 reactive scattering on Si(100)-2×1 (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2277-2288 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have simulated via molecular dynamics the interaction of F2 with the clean Si(100)-2×1 reconstructed surface. Using a Stillinger–Weber-type many-body potential with the Si–F interactions refit to ab initio data, we find that both vibrational and translational excitation of the incident F2 can lead to increased reactivity, but they do so in different ways. The dominant reaction channels are (a) F-atom abstraction, leading to the formation of one Si–F bond while the remaining fluorine atom is ejected away from the surface, and (b) dissociative chemisorption, where both fluorine atoms in the incident F2 molecule form Si–F bonds on the surface. Nonreactive scattering is almost never observed. As a result, enhanced reactivity is mainly characterized by an increase in dissociative chemisorption at the expense of F-atom abstraction and by a corresponding increase in the initial reaction probability S0. We find S0 ranges from 0.57±0.04 for the lowest excitation energies to 0.78±0.04 for the largest translational excitation of 20.9 kcal/mol. For cases where F-atom abstraction occurs, the exit velocities of fluorine atoms ejected from the surface are found to be independent of the incident F2 energy and with kinetic temperatures much higher than the surface temperature, suggesting that the exiting fluorine atom does not equilibrate with the surface, yet loses memory of its initial state. Finally, for dissociative chemisorption trajectories, we find that the adsite location of the two fluorine atoms is strongly dependent on the incident orientation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466526
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  • 18
    Electronic Resource
    Electronic Resource
    Pseudospectral multireference single and double excitation configuration interaction (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 7564-7572 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a pseudospectral formulation of the multireference single and double excitation configuration interaction method. A formal scaling advantage is achieved and practical timings are presented. The accuracy of the pseudospectral approximation within this method is probed for a variety of test cases. The method is typically accurate to within 1 mhartree while being up to six times faster than conventional codes. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469088
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  • 19
    Electronic Resource
    Electronic Resource
    Pseudospectral Møller–Plesset perturbation theory through third order (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 3631-3638 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a formulation and implementation of Møller–Plesset perturbation theory in a pseudospectral framework. At the second-order level, the pseudospectral formulation is a formally a factor of N/n faster than conventional approaches, while the third order is formally faster by a factor of n, where N is the number of atomic orbitals and n is the number of occupied orbitals. The accuracy of the resulting energies is probed for a number of test cases. Practical timings are presented and show conclusively that the pseudospectral formulation is faster than conventional ones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466350
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  • 20
    Electronic Resource
    Electronic Resource
    Solvation dynamics for an ion pair in a polar solvent: Time-dependent fluorescence and photochemical charge transfer (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 5961-5979 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The results of a molecular dynamics (MD) computer simulation are presented for the solvation dynamics of an ion pair instanteously produced from a neutral pair, in a model polar aprotic solvent. These time-dependent fluorescence dynamics are analyzed theoretically to examine the validity of several linear response theory approaches, as well as of various theoretical descriptions (e.g., Langevin equation) for the solvent dynamics per se. It is found that these dynamics are dominated for short times by a simple inertial Gaussian behavior, a feature which is absent in many current theoretical treatments, and which is related to the approximate validity of linear response theory. Nonlinear aspects, such as an overall spectral narrowing, but a transient initial spectral broadening, are also discussed. A model photochemical charge transfer process is also briefly considered to elucidate aspects of the connection between solvation dynamics and chemical kinetic population evolution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460431
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