Showing papers by "Gustavo E. Scuseria published in 1991"

Analytic evaluation of energy gradients for the singles and doubles coupled cluster method including perturbative triple excitations: Theory and applications to FOOF and Cr2

Abstract: The analytic energy gradient for the singles and doubles coupled cluster method including a perturbative correction due to triple excitations [CCSD(T)] is formulated and computationally implemented. Encouraged by the recent success in reproducing the experimental equilibrium structure and vibrational frequencies of ozone, the new CCSD(T) gradient method is tested with two other ‘‘difficult’’ quantum chemistry problems: FOOF and Cr2. With the largest basis set employed in this work [triple zeta plus two sets of polarization functions (TZ2Pf)] at the CCSD(T) level of theory, the predictions for the O–O and O–F bond lengths in FOOF are 1.218 and 1.589 A, respectively. These figures are in good agreement with the experimental values 1.216 and 1.575 A. Based on CCSD calculations with even larger basis sets, it is concluded that the error of 0.014 A in the O–F bond length at the TZ2Pf/CCSD(T) level of theory is due to the remaining basis set deficiency. On the other hand, the CCSD(T) prediction for the equilibrium bond length of Cr2 (1.604 A), obtained with a large (10s8p3d2f1g) basis set capable of achieving the Hartree–Fock limit, is still 0.075 A shorter than experiment, clearly indicating the importance of higher than connected triple excitations in a single‐reference treatment of this particular problem.

The dipole moment of carbon monoxide

Abstract: The dipole moment of CO has been calculated with many‐body perturbation theory (MBPT) and coupled cluster (CC) methods using basis sets which have been optimized at the MBPT‐2 level. It is demonstrated that triple excitations as well as g‐type functions in the basis set are crucial to obtain satisfactory agreement with experiment. The most reliable prediction (0.125 D) is obtained at the CCSD(T) (coupled cluster including all single, double, and connected triple excitations, perturbatively) level of theory using a 10s9p4d2f1g basis set (160 basis functions). This result is in excellent agreement with the experimental value of 0.122 D.

Ab initio theoretical study of small GaAs clusters

Abstract: Theoretical calculations for the closed‐shell ground state of small GaxAsy clusters (x=y; x=2–4) are carried out at the self‐consistent field (SCF) Hartree–Fock level of theory, using analytic energy gradients for rapid geometry optimization. In addition, for Ga2As2, the SCF results are compared with theoretical predictions obtained at the coupled cluster level of theory including all single, double and perturbative triple excitations, CCSD(T). The equilibrium structures for Ga2As2, Ga3As3, and Ga4As4 are found to be of D2h, C1, and Ci symmetry, respectively. The cluster binding energies with respect to GaAs dimers (3Σ−) and Ga(2P)+As(4S) atoms are also obtained.

A coupled cluster study of the classical barrier height of the F+H2→FH+H reaction

Abstract: Basis sets as large as F[7s7p5d4f2g]/H[6s5p4d2f] have been used in connection with the coupled cluster method including all single, double, and perturbative triple excitations [CCSD(T)] to predict the classical barrier height of the F+H2→FH+H reaction. Employing transition state geometries optimized at the seven‐ and nine‐electron CCSD(T) level with a [5s5p3d2f1g/4s3p2d1f] basis set, the calculated nine‐electron barrier of 2.05 kcal/mol exhibits a 0.20 kcal/mol correlation contribution from the fluorine 2s electrons. Comparison with full configuration interaction (FCI) calculations in smaller basis sets for the electron affinity (EA) of fluorine, the energy of FH at stretched bond lengths, and the barrier height itself, demonstrate that the CCSD(T) method is capable of accurately reproducing the exact benchmark results. Employing large atomic natural orbital basis sets and correlating the F 2s electrons, the CCSD(T) predictions for the electron affinity (EA) of F (3.36 eV), the spectroscopic constants of ...

The Dipole Moment of Carbon Monoxide

Abstract: The dipole moment of CO has been calculated with many‐body perturbation theory (MBPT) and coupled cluster (CC) methods using basis sets which have been optimized at the MBPT‐2 level. It is demonstrated that triple excitations as well as g‐type functions in the basis set are crucial to obtain satisfactory agreement with experiment. The most reliable prediction (0.125 D) is obtained at the CCSD(T) (coupled cluster including all single, double, and connected triple excitations, perturbatively) level of theory using a 10s9p4d2f1g basis set (160 basis functions). This result is in excellent agreement with the experimental value of 0.122 D.

Ab initio theoretical study of the ethynyl radical + carbon monoxide reaction and the HC2CO radical

Abstract: The ethynyl radical (HC{sub 2}) importance stems from its intermediate role in a number of environments. It figures prominently as a precursor in the soot formation of combustion processes. The ethynyl radical, carbon monoxide, and their product (HC{sub 2}CO) have been studied at the self-consistent field (SCF), configuration interaction, and coupled cluster correlation level including all single and double excitations (CISD and CCSD, respectively). The basis sets employed were of double {integral} plus polarization (DZP) and triple {integral} plus double polarization (TZ2P) quality. The geometries were optimized at both the SCF and CISD levels of theory. The energetics of the reaction were also studied by employing the open-shell coupled cluster method. A nonlinear HC{sub 2}CO product was found to be a minimum while the linear HC{sub 2}CO structure proved to be a stationary point with two imaginary frequencies. Our best theoretical prediction after correction for basis set superposition error and zero-point vibrational energy yields for HC{sub 2}Co a binding energy of 20.0 kcal/mol with a very small (0.8 kcal/mol) activation barrier. Vibrational frequencies are also presented.

Application of open-shell coupled cluster theory to the ground state of GaAs

Abstract: Theoretical calculations at the coupled cluster level of theory including all single, double and perturbative triple excitations, CCSD(T), are carried out for the3Σ− ground state of GaAs. Employing a (7s5p3d1f) basis set, the theoretical predictions forre (2.560 A), ω e (217 cm−1),De (1.84 eV), and IP (7.80 eV), are in good agreement with recent experimental results. The importance of includingf-type polarization functions in the basis set and the effect of correlating 3d electrons are discussed in detail.