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  • 1
    Electronic Resource
    Electronic Resource
    On the systematic improvement of fixed-node diffusion quantum Monte Carlo energies using pair natural orbital CI guide functions (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 8457-8463 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: While the diffusion quantum Monte Carlo method (DQMC) is capable, in principle, of calculating exact ground state energies, in practice the fixed-node (FN) approximation leads to node location errors which make FN-DQMC energies upper bounds. It is shown that the node location error can be reduced systematically and without prohibitive increase of computer time requirements by using nodes derived from pair natural orbital CI wave functions (PNO-CI). The reduction is demonstrated for the N atom and the molecules N2 and H2O. With the DQMC/PNOCI method, we obtain a variational energy of −109.520(3) H for the N2 molecule and −76.429(1) H for the ground state of the water molecule which is only 22 and 9 mH above the estimated nonrelativistic ground state energy, respectively. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1318748
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  • 2
    Electronic Resource
    Electronic Resource
    On the accuracy of the fixed-node diffusion quantum Monte Carlo method (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 5362-5366 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The accuracy of the fixed-node diffusion quantum Monte Carlo (FN-DQMC) method is compared to the coupled cluster method CCSD(T). For a test set of 20 small molecules and 17 reactions the electronic contribution to the reaction enthalpy is calculated with the FN-DQMC method using the nodes of a Slater determinant calculated at the HF/cc-pVTZ level. By comparison with reference reaction enthalpies the FN-DQMC method is shown to be more accurate than the CCSD(T)/cc-pVDZ method and almost as accurate as CCSD(T)/cc-pVTZ. The deviation from the reference data is comparable to the CCSD(T)/cc-pVTZ deviation, but, with only two exceptions, of opposite sign. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1394757
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  • 3
    Electronic Resource
    Electronic Resource
    Accurate quantum Monte Carlo calculations for hydrogen fluoride and the fluorine atom (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 4636-4640 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum Monte Carlo calculations of the ground state energies of the fluorine atom and the hydrogen fluoride molecule have been carried out using both fixed-node and released-node methods. The fixed-node total energies for both F and FH are among the most accurate available. The fixed-node dissociation energy De=141.3(4) kcal mol−1 is in excellent agreement with experimental data. The cancellation of the nodal error was achieved with trial wave functions having nodal surfaces determined by Slater determinants of the near Hartree–Fock limit quality. With the released-node Green's function quantum Monte Carlo method the lowest upper bounds to the ground state energies to date for both F [E0=−99.731(4) Hartree] and FH [E0=−100.458(5) Hartree] have been obtained. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472306
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  • 4
    Electronic Resource
    Electronic Resource
    Improved estimates of the total correlation energy in the ground state of the water molecule (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 7706-7708 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two new calculations of the electronic energy of the ground state of the water molecule yield energies lower than those of any previously reported variational calculations. A fixed-node quantum Monte Carlo calculation gives −76.420(1) hartrees and an analytic variational calculation gives −76.4274 hartrees. These values lie only 17 and 11 mhartrees, respectively, above the "experimental" value.© 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473770
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  • 5
    Electronic Resource
    Electronic Resource
    First-row hydrides: Dissociation and ground state energies using quantum Monte Carlo (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 7573-7578 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate ground state energies comparable to or better than the best previous ab initio results can be obtained using the fixed-node quantum Monte Carlo (FN-DQMC) method. The residual energy, the nodal error due to the error in the nodal structure of a trial wave function, is examined in this study using nodal surfaces given by near HF-limit wave functions. The study is aimed at better understanding of the nodal error and the cancellation of nodal errors in calculating energy differences. Calculations have been carried out for the first-row hydrides LiH to FH and the corresponding atoms. The FN-DQMC ground state energies are among the lowest to date. The dissociation energies De have been calculated with accuracies of 0.5 kcal mol−1 or better. For all hydrides, the dissociation energies are consistent with experimental values. The fixed-node quantum Monte Carlo method can therefore offer a very straight-forward way to calculate highly accurate dissociation energies. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472584
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  • 6
    Electronic Resource
    Electronic Resource
    Accurate nonrelativistic energies for 2Pe states of the Li isoelectronic series (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 62 (1997), S. 77-88 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Highly accurate upper bounds for several 2Pe states of the Li isoelectronic series obtained by extensive Hylleraas-Cl calculations are given. The best value for the 22Pe state (1s2p2) of Li is -5.21373920 au. The evaluation of the occurring integrals is given explicitly. Additionally, we present some expectation values and isotope energies of the Li isoelectronic series. © 1997 John Wiley & Sons, Inc.
    Additional Material: 11 Tab.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Upper- and lower-bound Hylleraas-CI calculations for the nonrelativistic Po states of the 4He isotope (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 25-30 
    Details
    ISSN: 0020-7608
    Keywords: Hylleraas-CI ; P states of 4He ; variance minimization ; upper and lower bounds ; perturbation theory ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Extensive Hylleraas-CI calculations for the lowest Po states of 4He were performed. The dependence of the variational energy values Eκ on the mass parameter κ given by κ=mHe2+/me- is discussed. Furthermore, lower bounds to Eκ were calculated using variance minimization.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 25-30, 1998
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Accurate nonrelativistic energies for 2Po states of the Li isoelectronic series (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 107-116 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: With large Hylleraas-configuration interaction (CI) basis sets highly accurate upper bounds for the lowest 2Po states of the Li isoelectronic series up to Ne are given. The corresponding Hamiltonian H and the operator Σi 〈 j, ▿i ▿j are transformed into nonorthogonal coordinates expressed in interparticle and angular terms. The evaluation of the occurring integrals is reduced to the calculation of well-known auxiliary integrals. Furthermore some expectation values and isotope energies are calculated. The isotope energies are obtained using perturbation theory in first-order approximation. © 1997 John Wiley & Sons, Inc.
    Additional Material: 10 Tab.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Nonrelativistic energies for the Be atom: Double-linked Hylleraas-CI calculation (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 241-247 
    Details
    ISSN: 0020-7608
    Keywords: Hylleraas-CI ; Be atom ; explicitly correlated wave function ; ground-state energy ; double-linked basis set ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hylleraas-configuration interaction (CI) calculations have been carried out with double-linked basis sets for the Be atom. Our best upper bound for the 1S ground state is E0 = -14.6673547Eh. Furthermore, upper bounds for the two lowest excited 1S states have been calculated.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 241-247, 1998
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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