| 1. |
- Lindh, Roland, 1958-, et al.
(författare)
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Benzyne thermochemistry : A benchmark ab initio study
- 1999
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 103:48, s. 9913-9920
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Tidskriftsartikel (refereegranskat)abstract
- The thermochemistry of the benzynes has been reinvestigated in a set of benchmark calculations including, e.g., multireference perturbation theory in combination with large basis sets up to correlation consistent polarized valence quadruple xi followed by basis set extrapolation procedures. The vibrational corrections have been deduced from multiconfiguration self-consistent field (MCSCF) calculations employing average atomic natural orbital basis sets. The quality of the isodesmic reactions has been investigated by analyzing the errors of the utilized methods in predicting the CH bond strengths and energies of the related molecules. It turns out that multireference third order perturbation theory, although occasionally better than second order, suffers from less systematic errors and thus is not as well suited for use in isodesmic reactions as the corresponding second order theory. The present extended calculations show that all the isodesmic reactions used in previous studies of the thermochemistry of the benzynes are adequate. Furthermore, it is demonstrated that multireference second order perturbation theory accurately reproduces the singlet-triplet energy splittings of the benzynes.
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| 2. |
- Lindh, Roland, 1958-, et al.
(författare)
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Force-constant weighted redundant coordinates in molecular geometry optimizations
- 1999
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Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 303:5-6, s. 567-575
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Tidskriftsartikel (refereegranskat)abstract
- A novel procedure to select internal coordinates for molecular geometry optimizations is presented. The procedure has features in common with other so-called redundant internal coordinates schemes. It is a black-box method which automatically selects an appropriate set of internal coordinates in which the geometry optimization is performed. The method is explicitly expressed in the non-redundant parameter space, thus avoiding the need for projections from the redundant internal coordinate space. The new procedure introduces a weighting in which the redundancy is modified prior to the generation of the non-redundant internal coordinates. The new method favors those redundant internal coordinates which are the most significant. It has favorable properties for the automatic generation of molecular coordinates in van der Waals complexes and transition state optimizations.
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| 3. |
- Lindh, Roland, et al.
(författare)
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On the significance of the trigger reaction in the action of the calicheamicin γI 1 anti-cancer drug
- 1997
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Ingår i: Theoretical Chemistry Accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 97:1, s. 203-210
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Tidskriftsartikel (refereegranskat)abstract
- The significance of the so-called trigger reaction in the reaction mechanism of the calicheamicin γI 1 anti-cancer drug has been studied with ab initio quantum chemical methods. The structures of four fragments of calicheamicin γI 1, consisting of either 39 or 41 atoms, have been fully optimized using the Becke-Perdew86 density functional method and the 6-31G* basis sets. The four structures constitute members of an isodesmic reaction for which the reaction energy is a direct measure of the change in activation energy of the Bergman reaction, caused by the structural rearrangements of the preceding trigger reaction. This difference in activation energy has been calculated with density functional theory, using the exchange-correlation functional mentioned above, and with second-order Møller-Plesset perturbation theory (MP2), employing an ANO-type basis set. In both cases a value of 12 kcal/ mol is obtained. The study firmly supports the hypothesis that the significance of the trigger reaction is to saturate a double bond in the vicinity of the enediyne group, which counteracts the formation of the biradical state of the drug. The MP2 computations became feasible by a novel implementation of an integral-direct, distributed-data, parallel MP2 algorithm.
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| 4. |
- Lindh, Roland, 1958-, et al.
(författare)
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Singlet benzyne thermochemistry : A CASPT2 study of the enthalpies of formation
- 1996
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 258:3-4, s. 409-415
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Tidskriftsartikel (refereegranskat)abstract
- The enthalpies of formation for singlet benzynes were examined at the CASPT2 (second order perturbation theory, based on a complete active space SCF reference function) level of theory, The study employed isodesmic reactions in order to minimize the influence of systematic errors of the method. The results obtained for m- and p-benzyne were found to dispute earlier experimentally deduced values; a discrepancy of about 5 kcal/mol was observed. As a consequence, the enthalpy of reaction of the so called Bergman reaction, deduced previously from the experimental enthalpy of formation of p-benzyne, seems to be overestimated by about 5 kcal/mol. Furthermore, it was observed that the CCSD(T) method (coupled cluster singles and doubles with a perturbatative estimate of the triples contribution) is inappropriate for a balanced treatment of the Bergman reaction, due to the extensive differences in the electronic structures of the two reactants.
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| 5. |
- Marques, F. M., et al.
(författare)
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Neutrons from the breakup of C-19
- 1996
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Ingår i: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 381:4, s. 407-412
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Tidskriftsartikel (refereegranskat)abstract
- Neutrons arising from the breakup of a 30 MeV/nucleon C-19 beam on a tantalum target have been measured using the 98 element array DEMON. A narrow, forward peaked neutron angular distribution, with a corresponding momentum spread considerably smaller than those measured simultaneously for N-21, O-22 and F-24 was observed for charged fragments with Z
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| 6. |
- Schütz, Martin, et al.
(författare)
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An integral direct, distributed-data, parallel MP2 algorithm
- 1997
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Ingår i: THEORETICA CHIMICA ACTA. - 0040-5744. ; 95:1-2, s. 13-34
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Tidskriftsartikel (refereegranskat)abstract
- A scalable integral direct, distributed-data parallel algorithm for four-index transformation is presented. The algorithm was implemented in the context of the second-order Moller-Plesset (MP2) energy evaluation, yet it is easily adopted for other electron correlation methods, where only MO integrals with two indices in the virtual orbitals space are required. The major computational steps of the MP2 energy are the two-electron integral evaluation O(N-4) and transformation into the MO basis O(ON4) where N is the number of basis functions, and O the number of occupied orbitals, respectively. The associated maximal communication costs scale as O(n(Sigma)O(2)V N), where V and n(Sigma) denote the number of virtual orbitals, and the number of symmetry-unique shells. The largest local and global memory requirements are Co(N-2) for the MO coefficients and O(OV N) for the three-quarter transformed integrals, respectively. Several aspects of the implementation such as symmetry-treatment, integral prescreening, and the distribution of data and computational tasks are discussed. The parallel efficiency of the algorithm is demonstrated by calculations on the phenanthrene molecule, with 762 primitive Gaussians, contracted to 412 basis functions. The calculations were performed on an IBM SP2 with 48 nodes. The measured wall clock time on 48 nodes is less than 15 min for this calculation, and the speedup relative to single-node execution is estimated to 527. This superlinear speedup is a result of exploiting both the compute power and the aggregate memory of the parallel computer. The tatter reduces the number of passes through the AO integral list, and hence the operation count of the calculation. The test calculations also show that the evaluation of the two-electron integrals dominates the calculation, despite the higher scaling of the transformation step.
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| 7. |
- Schütz, Martin, et al.
(författare)
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Integral-direct electron correlation methods
- 1999
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Ingår i: Molecular Physics. - 0026-8976 .- 1362-3028. ; 96:4, s. 719-733
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Tidskriftsartikel (refereegranskat)abstract
- Integral-direct implementations of Moller-Plesset perturbation theory up to fourth order (MP4(SDQ) without triple excitations), quadratic configuration interaction with single and double excitations (QCISD), coupled cluster (CCSD), multiconfiguration self-consistent field (MCSCF), second- and third-order multireference perturbation theory (MRPT2, MRPT3), and internally contracted multireference configuration interaction (MRCI) methods are described. All methods use the same three integral-direct kernels, namely direct calculation of Fock matrices, direct partial integral transformation, and direct evaluation of external exchange matrices. New algorithms for the latter two procedures are proposed and implemented, for which the memory requirement scales only linearly with basis set size. Test calculations with more than 500 basis functions are presented. It is demonstrated that integral prescreening can significantly reduce the scaling of the computational cost with molecular size N. For small polyglycine peptides (Gly)(n) (n = 1-4), we observe a scaling of N-3 for the direct integral transformation (formal scaling N-5), and N-4 for the direct computation of the external exchange matrices (formal scaling N-6).
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| 8. |
- Schütz, Martin, et al.
(författare)
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The water dimer interaction energy : Convergence to the basis set limit at the correlated level
- 1997
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 107:12, s. 4597-4605
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Tidskriftsartikel (refereegranskat)abstract
- The water dimer interaction energy and its convergence to the basis set limit was investigated, with electron correlation effects treated at the level of second order Moller-Plesset perturbation theory (MP2). ANO-type and large uncontracted basis sets were used, spreading over a wide range in size; the biggest set included 1046 functions with angular momentum up to (l=7). Core correlation effects were treated accurately by augmenting the original valence basis with extended sets of core polarization functions. The MP2 dimer interaction energy at the basis set limit was determined to -4.94+/-0.02 kcal/mol, with a contribution due to core correlation of -0.04 kcal/mol. Furthermore, based on some elementary considerations from intermolecular perturbation theory, a simple procedure was devised, which brings the counterpoise corrected interaction energies of moderate basis set calculations closer to the basis set limit. The interaction energies so obtained turned out be surprisingly stable with respect to extensions of the basis set.
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