| 1. |
- Holt, Asbjörn, et al.
(författare)
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A NEMO potential that includes the dipole-quadrupole and quadrupole-quadrupole polarizability
- 2010
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Ingår i: Journal of Computational Chemistry. - : Wiley. - 0192-8651 .- 1096-987X. ; 31:8, s. 1583-1591
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Tidskriftsartikel (refereegranskat)abstract
- To increase the accuracy of molecular force fields a systematical and balanced improvement of the various terms included is needed. In this work, we have followed this strategy to improve the quality of the NEMO potential for the formaldehyde dimer by introducing local quadrupole moments and higher-order polarizabilities. It is found that inclusion of the quadrupole moment significantly improves the interaction potential. Furthermore, the inclusion of higher-order polarizabilities up to quadrupole-quadrupole polarizability is shown to give a better description of the intermolecular interaction. In addition, it is demonstrated that localized properties based on MP2 densities reproduces the BSSE corrected MP2 interaction energy at large intermolecular separations. This is not the case for HF-SCF based properties.
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| 2. |
- Holt, Asbjörn, et al.
(författare)
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An intramolecular induction correction model of the molecular dipole moment.
- 2008
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Ingår i: Journal of Computational Chemistry. - : Wiley. - 1096-987X .- 0192-8651. ; 29:7, s. 1084-1091
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Tidskriftsartikel (refereegranskat)abstract
- A model for intramolecular polarization is presented. It is used to describe the changes in the molecular charge distribution occurring as a response to changes of dihedral angles in the molecule. The model is based on a multicenter multipole distribution of the molecular charge distribution. The electric field from this charge distribution induce dipole moments in the same molecule. The model contains atom type parameters to describe the damping of the electric field. A total of four atom types are used. The parameters are fitted to a calibration set with various functional groups, and tested against a validation set. The error obtained for the calibration set is reduced by 92% and by 88% for the validation set, if compared to an accurate state-of-the-art force field. It is shown that rotating the non-polarizable multicenter multipole distribution for the equilibrium geometry gives too large dipole moments for dihedral angles deviating from the equilibrium geometry. This will lead to too large long-range attractions in simulations. This problem is overcome by using the dipole polarizability correction suggested here, which gives dipole moments very close to the Hartree-Fock dipole moments obtained from reference calculations. (c) 2007 Wiley Periodicals, Inc. J Comput Chem 2007.
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| 3. |
- Holt, Asbjörn, et al.
(författare)
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Calculation of the Molecular and Atomic Properties of Selected Anions in Water.
- 2011
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 115:5, s. 1098-1104
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Tidskriftsartikel (refereegranskat)abstract
- The polarizability and other properties have been studied for F(-), Cl(-), Br(-), and HCOO(-) in water using a combined quantum chemical statistical mechanics simulation model that explicitly takes into account the Pauli repulsion as well as the electrostatic coupling between the QM system and the classical surroundings. It is shown that the surrounding molecules significantly reduce both the polarizability and the size of the anions. For the formate ions, local properties have been computed.
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| 4. |
- Holt, Asbjörn, et al.
(författare)
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Improvement of the NEMO Potential by Inclusion of Intramolecular Polarization
- 2009
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Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 109:6, s. 1255-1266
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Tidskriftsartikel (refereegranskat)abstract
- The intermolecular electrostatic and induction energy of a NEMO potential which includes intramolecular polarization, are compared with a regular NEMO potential and a NEMO potential with configurationally dependent local properties. It is shown that the inclusion of intramolecular polarization improves the description of the intermolecular interactions when using the NEMO potential. (c) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109: 1255-1266, 2009
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| 5. |
- Holt, Asbjörn, et al.
(författare)
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Inclusion of the quadrupole moment when describing polarization. The effect of the dipole-quadrupole polarizability.
- 2008
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Ingår i: Journal of Computational Chemistry. - : Wiley. - 1096-987X .- 0192-8651. ; 29:12, s. 2033-2038
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Tidskriftsartikel (refereegranskat)abstract
- A method to compute distributed dipole-quadrupole polarizabilities is suggested. The method is based on numerical differentiation of distributed quadrupole moments, using finite field perturbation calculations. It is tested using two different multicenter multipole expansions, and compared with results using polarizabilities obtained via the uncoupled Hartree-Fock approximation. The accuracy of these dipole-quadrupole polarizabilities are tested for different molecules and basis sets, by comparing the induced electrostatic potential of the Hartree-Fock density with the induced electrostatic potential of the polarization models. This is done by perturbing the molecules with an external homogeneous field and with an external dipole. It is found that inclusion of the dipole-quadrupole polarizability significantly improves the accuracy of the response of the molecule to these external perturbations. This suggests that inclusion of higher-order induced moments can be of importance when improving the description of intermolecular interactions using force fields. (c) 2008 Wiley Periodicals, Inc. J Comput Chem 2008.
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| 6. |
- Holt, Asbjörn, et al.
(författare)
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Induction correction model for rotation of two or three dihedral angles.
- 2008
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Ingår i: Journal of Computational Chemistry. - : Wiley. - 1096-987X .- 0192-8651. ; 29:12, s. 1905-1911
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Tidskriftsartikel (refereegranskat)abstract
- In a previous work we have introduced an intramolecular induction correction model. In this work we have used the model to calculate the total dipol moment of six molecules as a function of two or three dihedral angles that are simultaneously varied in the molecules. It is found that the induction model behaves very well for the systems studied when compared with a regular force field model where fixed charges and dipoles are rotated along with the atoms of the molecules. This suggests that the proposed induction correction model can be used to model systems containing several dihedral angles around which rotations are performed. (c) 2008 Wiley Periodicals, Inc. J Comput Chem, 2008.
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| 7. |
- Holt, Asbjørn
(författare)
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Modelling of polarization by molecular force fields: Further development of the NEMO potential
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding of intermolecular interactions are important in order to gain insight into a large number of physical processes. This thesis concerns itself with theoretical modelling of such interactions. In particular there is a focus one of interaction contributions called polarization. This interaction is sometimes ignored, but is a real and often important effect. For a model to be able to describe situations with varying chemical environments (for instance at surfaces) it must be included. The research presented in this thesis can be divided into three different categories; modelling of intramolecular polarization, modeling including higher-order polarizabilities and description of the chemical bond using a charge capacitance. Of these three categories the first two are concerned with improving the description of intermolecular interactions whereas the last part is concerned with the chemical bond. For intramolecular polarization a model based on previous work (J. Phys. Chem., 100:6950, 1996) is extended to include a damping. It is shown that this model improves the description of the molecular dipole moment when rotating dihedral angles, and that the model gives better interaction energies. In the study of higher-order polarizabilites a method for inclusion of localized dipole-quadrupole polarizabilities are presented, and it is shown that inclusion of this polarizability gives a better description of the induced potential around a molecule. In a second study localized dipole-quadrupole and quadrupole - quadrupole polarizabilities are included, and it is shown that inclusion of these gives better potential energy curves for the formaldehyde dimer. It is also shown that using local multipoles based on MP2 densities gives a correct behavior at larger separations. In the last research papers the charge capacitance presented and studied. The charge capacitance is based on the charge part of the localized dipole-dipole polarizability in the LoProp method. It is demonstrated that the charge capacitance is corresponds well with classical bonding situations. Furthermore it is shown that the charge capacitance is correlated with the strength of the agostic interaction in a series of molecules. It is shown that the charge capacitance correlates with other descriptors of bond strength in molecules containing carbon - carbon, carbon - silicon and silicon - silicon bonds. It is also demonstrated that the charge capacitance can be used, together with other descriptors, to predict the regio-selectivity of aromatic nucleophilic substitution
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| 8. |
- Holt, Asbjörn, et al.
(författare)
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The Charge Capacitance of the Chemical Bond: Application to Bonds Containing Metals
- 2009
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Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608. ; 109:3, s. 618-628
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Tidskriftsartikel (refereegranskat)abstract
- The charge capacitance of metal containing complexes are studied. For molecules with multiple bonding between the metal atoms it is found that the charge capacitance is correlated to the maximum bond order, natural bond order, and to some extent the effective bond order. Furthermore the charge capacitance of some methylidene metal dihydride complexes are studied. These molecules have agostic interactions of varying strength, and it is concluded that this strength is very well reflected in the charge capacitances of the systems. In accordance with the definition of agostic interactions it is therefor concluded that the charge capacitance holds information about the strength of covalent interactions. The effect therefore on the agostic interactions upon substitution of one of the hydrogen atoms with fluorine in the methylidene metal complexes is studied, and found to reduce the agostic interactions. It is also demonstrated that there is an agostic interaction in an ArCrCrAr complex. The distance dependence of the charge capacitance is also discussed. (C) 2008 Wiley Periodicals, Inc. Int J Quantum Chem 109: 618-628, 2009
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| 9. |
- Holt, Asbjörn, et al.
(författare)
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The charge capacity of the chemical bond
- 2007
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 436:1-3, s. 297-301
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Tidskriftsartikel (refereegranskat)abstract
- A bond capacitance is introduced via the LoProp formalism, and the calculated bond capacitances reflects the properties of the corresponding chemical bond remarkably well. It is clearly possible to characterize the properties of the chemical bond with the help of the calculated bond capacitances.
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