| 1. |
- Bernhardsson, Anders, et al.
(författare)
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A direct implementation of the second-order derivatives of multiconfigurational SCF energies and an analysis of the preconditioning in the associated response equation
- 1999
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Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 96:4, s. 617-628
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Tidskriftsartikel (refereegranskat)abstract
- A direct implementation of the reduced multiplication scheme of the Rys-Gauss quadrature in the computation of the second-order geometric derivatives for MCSCF wavefunctions is presented. Characteristics unique to this implementation are simultaneous evaluation of first- and second-order two-electron integral derivatives, compact representation of intermediate elements and efficient prescreening. Finally, a novel technique for the preconditioning of the response equations is reported. Applications sizing up to 510 basis functions are included in the presentation.
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| 2. |
- Bernhardsson, Anders, et al.
(författare)
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Direct self-consistent reaction field with Pauli repulsion : Solvation effects on methylene peroxide
- 1996
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 251:3-4, s. 141-149
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Tidskriftsartikel (refereegranskat)abstract
- The properties of methylene peroxide in the gas phase and water have been examined. The solution effects have been modeled with a cavity. To make it possible to optimize the geometry of the cavity, we have added an approximate description of the Pauli repulsion between the solvent and the solute. An efficient direct implementation of the calculation of multicenter multipole operators is also described. We have applied the method to calculate the electric properties of methylene peroxide, a system that is characterized by its near degeneracy between a biradical and a zwitterionic state. As expected, it is shown that solvent effects will stabilize the ionic configuration, resulting in a considerably weakened OO bond and an increased double bond character in the CO bond. The theoretical study has been performed using the multiconfigurational (CAS) SCF method and second-order perturbation theory (CASPT2).
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| 3. |
- Engkvist, Ola, et al.
(författare)
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On the relation between retention indexes and the interaction between the solute and the column in gas-liquid chromatography
- 1996
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Ingår i: Journal of chemical information and computer sciences. - : American Chemical Society (ACS). - 0095-2338 .- 1520-5142. ; 36:6, s. 1153-1161
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Tidskriftsartikel (refereegranskat)abstract
- Gas-liquid chromatography retention indexes for organic molecules are determined by the interaction between the molecule and the column liquid phase. In this article, a model for calculating the interaction energy between a molecule and a dielectric wall is developed. The model is at least to our knowledge the first attempt to predict retention indexes from the interaction between the molecules and the column. This approach to predict retention indexes is radically different from methods proposed before. Earlier predictions of the retention indexes have been done by a large number of descriptors, which were Linearly correlated to the retention indexes. The developed model has been tested for polycyclic aromatic hydrocarbons mainly with a molecular weight of 302. For the molecules with MW 302 the obtained correlation coefficient is 0.92. A somewhat simpler model is used to fit PAH with different MWs. A correlation coefficient of 0.998 is obtained if the retention indexes were fitted to the logarithm of the interaction energies between the PAHs and the column.
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| 4. |
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| 5. |
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| 6. |
- 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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| 7. |
- 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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| 8. |
- Lindh, Roland, 1958-, et al.
(författare)
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On the use of a Hessian model function in molecular geometry optimizations
- 1995
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Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 241:4, s. 423-428
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Tidskriftsartikel (refereegranskat)abstract
- When a molecular equilibrium geometry is determined by minimizing the energy by a quasi-Newton-Raphson method, the number of iterations required depends critically on the choice of an approximate molecular Hessian matrix. We find that a simple 15-parameter function of the nuclear positions gives a good choice for any molecule with atoms from the first three rows of the periodic table. This Hessian is used for ah initio geometry optimizations with the quasi-Newton-Raphson method, with or without update. The equilibrium geometries of 30 molecules, with a variety of sizes and symmetries, is obtained with the new scheme, which is shown to converge significantly faster than other methods.
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| 9. |
- Lindh, Roland, 1958-, et al.
(författare)
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Research Article Extended ab Initio and Theoretical Thermodynamics Studies of the Bergman Reaction and the Energy Splitting of the Singlet o-, m-, and p-Benzynes
- 1995
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 117:27, s. 7186-7194
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Tidskriftsartikel (refereegranskat)abstract
- The autoaromatization of (Z)-hex-3-ene-1,5-diyne to the singlet biradical p-benzyne has been reinvestigated by state of the art ab initio methods. Previous CCSD(T)/6-31G(d,p) and CASPT2[0]/ANO[C(5s4p2dlf)/H(3s2p)] calculations estimated the reaction heat at 298 K to be 8-10 and 3.9 +/- 3.2 kcal/mol, respectively. Recent NO- and oxygen-dependent trapping experiments and collision-induced dissociation threshold energy experiments estimate the heat of reaction to be 8.5 +/- 1.0 kcal/mol at 470 K (corrected to 9.5 +/- 1.0 kcal/mol at 298 K) and 8.4 +/- 3.0 kcal/mol at 298 K, respectively. New theoretical estimates at 298 K predict the values at the basis set Limit for the CCSD(T) and CASPT2[gl] methods to be 12.7 +/- 2.0 and 5.4 +/- 2.0 kcal/mol, respectively. The experimentally predicted electronic contribution to the heat of activation is 28.6 kcal/mol. This can be compared with 25.5 and 29.8 kcal/mol from the CASPT2[gl] and the CCSD(T) methods, respectively. The new study has a much larger one-particle basis set for the CCSD(T) method as compared to earlier studies. For the CASPT2 investigation the better suited CASPT2[gl] approximation is utilized. The original CASPT2 method, CASPT2[0], systematically favors open-shell systems relative to closed-shell systems. This was previously corrected empirically. The current study shows that the energy difference between CCSD(T) and CASPT2[gl] at the basis set limit is estimated to be 7 +/- 2 kcal/mol. The study also demonstrates that the estimated heat of reaction is very sensitive to the quality of the basis set. In particular CCSD(T)/6-31G(d,p) approach underestimates the basis set limit of the enthalpy by approximately 5 kcal/mol. Furthermore, the relative energies of the p-, m-, and o-benzynes are computed at the CASPT2[gl] and CCSD(T) levels of theory. These results help to explain the discrepancy between the two methods in the case of the Bergman reaction. The deficiency of the CASPT2 method is mainly attributed to the approximate way in which the dynamic correlation is included by perturbation theory. A similar sized error is attributed to the CCSD(T) method due to the approximate way in which near degeneracy effects are included. This combined CCSD(T) and CASPT2[gl] study indicates that the most recent experimental value of the p-benzyne-o-benzyne energy splitting is overestimated.
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| 10. |
- Nyman, Ulf, et al.
(författare)
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Chronic aortic dissection: stenting of aortic true lumen obliteration with late dynamic variations of both lumens
- 1999
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Ingår i: Cardiovascular and Interventional Radiology. - : Springer Science and Business Media LLC. - 1432-086X .- 0174-1551. ; 22:2, s. 135-149
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Tidskriftsartikel (refereegranskat)abstract
- Percutaneous endovascular techniques were used to treat an arteriovenous fistula (AVF) associated with pancreatic transplantation. A pancreatic transplant superior mesenteric artery-to-superior mesenteric-vein AVF was successfully embolized while flow to the pancreas transplant was preserved. The embolization was aided by the use of Guglielmi detachable coils and a detachable balloon. No complications were encountered. At 23 months follow-up, the patient is doing well with no recurrence.
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