| 1. |
- Barnes, Leslie A, et al.
(author)
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Bond length, dipole moment, and harmonic frequency of CO
- 1993
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 98:5, s. 3972-3977
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Journal article (peer-reviewed)abstract
- A detailed comparison of some properties of CO is given, at the modified coupled-pair functional, single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), using a variety of basis sets. With very large one-particle basis sets, the CCSD(T) method gives excellent results for the bond distance, dipole moment, and harmonic frequency of CO. In a [6s 5p 4d 3f 2g 1h] + (1s 1p 1d) basis set, the bond distance is about 0.005a0 too large, the dipole moment about 0.005 a.u. too small, and the frequency about 6 cm-1 too small, when compared with experimental results.
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| 2. |
- Barnes, Leslie A, et al.
(author)
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Structure and energetics of Cr(CO)6 and Cr(CO)5
- 1993
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 98:5, s. 3978-3989
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Journal article (peer-reviewed)abstract
- The geometric structure of Cr(CO)6 is optimized at the modified coupled-pair functional (MCPF), single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), and the force constants for the totally symmetric representation are determined. The geometry of Cr(CO)5 is partially optimized at the MCPF, CCSD, and CCSD(T) levels of theory. Comparison with experimental data shows that the CCSD(T) method gives the best results for the structures and force constants, and that remaining errors are probably due to deficiencies in the one-particle basis sets used for CO. The total binding energies of Cr(CO)6 and Cr(CO)5 are also determined at the MCPF, CCSD, and CCSD(T) levels of theory. The CCSD(T) method gives a much larger total binding energy than either the MCPF or CCSD methods. An analysis of the basis set superposition error (BSSE) at the MCPF level of treatment points out limitations in the one-particle basis used here and in a previous study. Calculations using larger basis sets reduce the BSSE, but the total binding energy of Cr(CO)6 is Still Significantly smaller than the experimental value, although the first CO bond dissociation energy of Cr(CO)6 is well described. An investigation of 3s3p correlation reveals only a small effect. In the largest basis set, the total CO binding energy of Cr(CO)6 is estimated to be 140 kcal/mol at the CCSD(T) level of theory, or about 86% of the experimental value. The remaining discrepancy between the experimental and theoretical value is probably due to limitations in the one-particle basis, rather than limitations in the correlation treatment. In particular, an additional d function and an f function on each C and 0 are needed to obtain quantitative results. This is underscored by the fact that even using a very large primitive set (1042 primitive functions contracted to 300 basis functions), the superposition error for the total binding energy of Cr(CO)6 is 22 kcal/mol at the MCPF level of treatment.
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| 3. |
- Barnes, Leslie A, et al.
(author)
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Symmetry breaking in O+4 : an application of the Brueckner coupled-cluster method
- 1994
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 223:3, s. 207-214
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Journal article (peer-reviewed)abstract
- A recent calculation of the antisymmetric stretch frequency for the rectangular structure of quartet O4+ using the QCISD(T) method gave a value of 3710 cin-1. This anomalous frequency is shown to be a consequence of symmetry-breaking effects, which occur even though the QCISD(T) solution derived from a delocalized SCF reference function lies energetically well below the two localized (symmetry-broken) solutions at the equilibrium geometry. The symmetry breaking is almost eliminated at the CCSD level of theory, but the small remaining symmetry-breaking effects are magnified at the CCSD (T) level of theory so that the antisymmetric stretch frequency is still significantly in error. The Brueckner coupled-cluster method, however, leads to a symmetrical solution which is free of symmetry-breaking effects, with an antisymmetric stretch frequency of 1322 cm-1, in good agreement with our earlier calculations using the CASSCF/CASSI method.
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| 4. |
- Lindh, Roland, 1958-, et al.
(author)
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Ab Initio Study of the Bergman Reaction : The Autoaromatization of Hex-3-ene-1,5-diyne
- 1994
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 116:11, s. 4963-4969
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Journal article (peer-reviewed)abstract
- The Bergman reaction of the hex-3-ene-1,5-diyne molecule (enediyne) to form the singlet biradical p-benzyne structure under ring closure has been studied by ab initio methods. The complete active space (CAS) SCF method was used for geometry optimizations at five points along the reaction path of the Bergman reaction. Subsequent calculations using the multireference second-order perturbation theory based on a CASSCF reference (CASPT2) established the energetics along the reaction path, especially for the reactant, transition state, and product. The energetics were further corrected for zero point vibrational energy at the CASSCF level of theory. The study incorporated four different basis sets: a double-zeta plus polarization (DZP), a triple-zeta plus double polarization (TZ2P), and two different average atomic natural orbital (ANO) basis sets. The study predicts the energy barrier to ring closure to be 25.0 +/- 3.1 kcal/mol and the enthalpy of reaction to be 4.9 +/- 3.2 kcal/mol. The latter value deviates significantly from the value estimated on the basis of group additivity and the Born-Haber cycle. The enediyne moiety is the main functional group of a new class of anticancer agents. The activation of the Bergman reaction in these drugs has been argued to be due to a small structural change in the terminal-terminal carbon distance of about -0.5 Angstrom originating from an epoxide conversion in the rest of the drug. The present study indicates that such a structural change is not sufficient for the Bergman autoaromatization to proceed at an appreciable rate. A new reaction path involving no biradical formation, resulting in the same products, is suggested.
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| 5. |
- Lindh, Roland, 1958-, et al.
(author)
-
Accurate abinitio calculations of the quadrupole-moment of acetylene : a combined study of basis set, correlation, and vibrational effects
- 1991
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 94:6, s. 4356-4368
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Journal article (peer-reviewed)abstract
- The quadrupole moment of acetylene has been studied at the multiconfiguration self-consistent field (MCSCF) and multireference single and double configuration interaction (MRSDCI) level of theory. At the MCSCF level the pi-CI complete active space SCF (CASSCF) and the valence-CI CASSCF were employed. The subsequent MRSDCI calculations were continued until the reference space included all configuration state functions (CSFs) of the MCSCF wave function with a coefficient larger than 0.01 [MRSDCI(0.01)]. The higher level basis sets in this study were all based on van Duijneveldt’s C(13s 8p) and H(6s) and extensions of that basis set. The study shows in a consistent way that both the one- and n-particle spaces are saturated at the highest level of theory. The study has revealed that in addition to the well known increase of the quadrupole moment due to the inclusion of polarizing functions in the basis (typically 0.20 a.u.), the inclusion of electronic correlation in the model wave function as well as vibrational corrections will decrease the quadrupole moment significantly more, -0.66, -0.49, and -0.36 a.u., for the correlation correction and zero-point correction for HCCH and DCCD, respectively. The most accurate computations predict the quadrupole moment of HCCH, including zero-point correction, to be 4.29 +/- 0.12 a.u., which discriminates the experimental estimates of 4.03 +/- 0.30, 4.28 +/- 0.30, and 4.57 +/- 0.30 a.u. (the first being the favored value). The quadrupole moment of DCCD is computed to 4.42 +/- 0.10 a.u. In the study it was observed that in contradiction to previous experiences the use of the model equilibrium geometries rather than the experimental geometry gives a smoother convergence as the level of theory is increased. The effects of basis set quality and electron correlation on the quadrupole moment are studied in detail. These effects are analyzed with reference to the redistribution of the electronic charge.
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| 6. |
- Lindh, Roland, 1958-, et al.
(author)
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The energy separation between the classical and nonclassical isomers of protonated acetylene : An extensive study in one‐ and n‐particle space
- 1991
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 94:12, s. 8008-8014
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Journal article (peer-reviewed)abstract
- The energy separation between the classical and nonclassical forms of protonated acetylene has been reinvestigated in light of the recent experimentally deduced lower bound to this value of 6.0 kcal/mol. The objective of the present study is to use state-of-the-art ab initio quantum mechanical methods to establish this energy difference to within chemical accuracy (i.e., about 1 kcal/mol). The one-particle basis sets include up to g-type functions and the electron correlation methods include single and double excitation coupled-cluster (CCSD), the CCSD(T) extension, multireference configuration interaction, and the averaged coupled-pair functional methods. A correction for zero-point vibrational energies has also been included, yielding a best estimate for the energy difference between the classical and nonclassical forms of 3.7 +/- 1.3 kcal/mol.
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| 7. |
- Lindh, Roland, 1958-, et al.
(author)
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The fraternal twins of quartet O+4
- 1994
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 100:1, s. 224-237
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Journal article (peer-reviewed)abstract
- Eleven stationary geometries of quartet O4+ have been studied by ab initio methods. The geometries were optimized at the complete active space self-consistent field (CASSCF) level of theory and the energies were calculated by the multiconfigurational second order pertubation method (CASPT2), using double-zeta plus polarization (DZP), triple-zeta plus double polarization (TZ2P), average atomic natural orbital (ANO) [5s4p2d] and average ANO [6s5p3d2f] basis sets. The rectangular and trans-planar structures are found to be the most stable, with an energy barrier to conversion between the two at the threshold of dissociation. Both have a delocalized hole and are stable relative to separated O2 and O2+ by 11.0 and 11. 5 kcal/mol for the rectangular and the trans-planar structure, respectively, compared with the experimentally deduced energy in the range of 9.2 to 10.8 kcal/mol. The adiabatic ionization potentials of O4 and O2 are computed to be 11.67 and 12.21 eV, while experimental values are 11.66 and 12.07 eV, respectively. The vibrational frequencies have been computed for all degrees of freedom at the CASSCF level of theory. Symmetry breaking is found to be a particular problem in the computation of the antisymmetric stretch frequency for the delocalized structures at the CASSCF level of theory. Attempts to rectify these problems using the restricted active space self-consistent field (RASSCF) method leads to additional difficulties, but further analysis yields insight into the symmetry breaking and problems with earlier calculations. Finally, a nonorthogonal configuration interaction (CI) calculation based on the interaction of localized CASSCF wave functions using the complete active space state interation (CASSI) method leads to a balanced treatment of the antisymmetric stretch which is free from symmetry breaking. The study explains the four most prominent absorption frequencies observed in the partially unassigned IR spectrum of 04+ isolated in solid neon as the antisymmetric OO stretch, and the combination band of the symmetric and antisymmetric OO stretch of both the rectangular and trans-planar structures.
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| 8. |
- Lindh, Roland, 1958-
(author)
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THE REDUCED MULTIPLICATION SCHEME OF THE RYS-GAUSS QUADRATURE FOR 1ST-ORDER INTEGRAL DERIVATIVES
- 1993
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In: THEORETICA CHIMICA ACTA. - 0040-5744. ; 85:6, s. 423-440
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Journal article (peer-reviewed)abstract
- An implementation of the reduced multiplication scheme of the Rys-Gauss quadrature to compute the gradients of electron repulsion integrals is discussed. The study demonstrates that the Rys-Gauss quadrature is very suitable for efficient utilization of simplifications as offered by the direct computation of symmetry adapted gradients and the use of the translational invariance of the integrals. The introduction of the so-called intermediate products is also demonstrated to further reduce the floating point operation count. Two prescreening techniques based on the 2nd order density matrix in the basis of the uncontracted Gaussian functions is proposed and investigated in the paper. This investigation gives on hand that it is not necessary to employ the Cauchy-Schwarz inequality to achieve efficient prescreening. All the features mentioned above were demonstrated by their implementation into the gradient program ALASKA. The paper offers a theoretical and practical assessment of the modified Rys-Gauss quadrature in comparison with other methods and implementations and a detailed analysis of the behavior of the method as suggested above as a function of changes with respect to symmetry, basis set quality, molecular size, and prescreening threshold.
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| 9. |
- Lindh, Roland, 1958-, et al.
(author)
-
The reduced multiplication scheme of the Rys quadrature and new recurrence relations for auxiliary function based two‐electron integral evaluation
- 1991
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 95:8, s. 5889-5897
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Journal article (peer-reviewed)abstract
- The reduced multiplication scheme of the Rys quadrature is presented. The method is based on new ways in which the Rys quadrature can be developed if it is implemented together with the transfer equation applied to the contracted integrals. In parallel to the new scheme of the Rys quadrature improvements are suggested to the auxiliary function based algorithms. The two new methods have very favorable theoretical floating point operation (FLOP) counts as compared to other methods. It is noted that the only significant difference in performance of the two new methods is due to the vectorizability of the presented algorithms. In order to exhibit this, both methods were implemented in the integral program SEWARD. Timings are presented for comparisons with other implementations. Finally, it is demonstrated how the transfer equation in connection with the use of spherical harmonic Gaussians offers a very attractive path to compute the two-electron integrals of such basis functions. It is demonstrated both theoretically and with actual performance that the use of spherical harmonic Gaussians offers a clear advantage over the traditional evaluation of the two-electron integrals in the Cartesian Gaussian basis.
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| 10. |
- Rendell, Alistair P, et al.
(author)
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Quantum chemistry on parallel computer architectures : oupled-cluster theory applied to the bending potential of fulminic acid
- 1992
-
In: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 194:1-2, s. 84-94
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Journal article (peer-reviewed)abstract
- We present a formulation of the singles and doubles coupled-cluster (CCSD) method suitable for distributed memory parallel computer architectures. Results from test calculations using up to 128 nodes of the Intel i860 hypercube are presented. Although the new algorithm is highly parallel, the asymptotic speed is quickly reached due to the input/output (IO) demands on the concurrent file system. To alleviate these IO problems, as well as disk storage requirements, we propose a “semi-direct” CCSD method whereby the atomic orbital integrals are evaluated in each iteration of the procedure in order to determine the contribution from certain terms. Using CCSD(T)/TZ2P optimized geometries, the calculations show that the Born-Oppenheimer ground-state potential energy surface yields a linear structure for fulminic acid. HCNO, in agreement with the semirigid bender analysis by Bunker, Landsberg and Winnewisser.
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| 11. |
- Ryu, U, et al.
(author)
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An efficient method of implementing the horizontal recurrence relation in the evaluation of electron repulsion integrals using Cartesian Gaussian functios
- 1991
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 185:5-6, s. 562-568
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Journal article (peer-reviewed)abstract
- The tree search problem is considered for optimizing the horizontal recurrence relation step, which is a three-dimensional recurrence relation used in generating two-electron integrals. By eliminating redundant work, it is possible to achieve 13%, 25%, 38% and 44% savings in floating point operations and 21%, 34%, 46% and 53% reductions in memory requirements for the (ff\textbackslashff), (gg\textbackslashgg), (hh\textbackslashhh) and (ii\textbackslashii) cases, respectively. These savings and reductions will lead to quite efficient two-electron integral and derivative programs when high angular momentum functions are included.
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| 12. |
- Serrano-Andrés, Luis, et al.
(author)
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Theoretical study of the electronic spectrum of all-trans-1,3,5,7-octatetraene
- 1993
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In: The journal of physical chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 97:37, s. 9360-9368
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Journal article (peer-reviewed)abstract
- Ab initio calculations have been performed for the electronic spectrum of all-trans-1,3,5,7-octatetraene including geometry optimizations for the ground and low-lying valence excited singlet states at the complete active space self-consistent-field level. Excitation energies have been calculated using multiconfigurational second-order perturbation theory (CASPT2). Computed excitation energies for the valence and a number of low-lying Rydberg excited singlet and triplet states make possible confident assignments of the main features reported in the experimental spectra. The computed vertical and nonvertical excitations and the fluorescence maxima confirm the suggested presence of the 2(1)A(g) valence state below the 1 1B(u) optically allowed state, as is the characteristic for the longer polyenes. The agreement is within 0.05 eV for all excitation energies where comparison with experimental data is possible.
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| 13. |
- Serrano-Andrés, Luis, et al.
(author)
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Towards an accurate molecular orbital theory for excited states : Ethene, butadiene, and hexatriene
- 1993
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 98:4, s. 3151-3162
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Journal article (peer-reviewed)abstract
- A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans-1,3-butadiene, and trans-trans-1,3,5-hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second order perturbation theory with the CASSCF wave function as the reference state. The approach is here tested in a calculation of the valence and Rydberg excited singlet and triplet states of the title molecules, using extended atomic natural orbital (ANO) basis sets. The ethene calculations comprised the two valence states plus all singlet and triplet Rydberg states of 3s, 3p, and 3d character, with errors in computed excitation energies smaller than 0.13 eV in all cases except the V state, for which the vertical excitation energy was about 0.4 eV too large. The two lowest triplet states and nine singlet states were studied in butadiene. The largest error (0.37 eV) was found for the 2 B-1(u) state. The two lowest triplet and seven lowest singlet states in hexatriene had excitation energies in error with less than 0.17 eV.
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