摘要:

The brace notation, introduced by Allen and Császár (1993,J. chem. Phys.,982983), provides a simple and compact way to deal with derivatives of arbitrary non-tensorial quantities. One of its main advantages is that it builds the permutational symmetry of the derivatives directly into the formalism. The brace notation is applied to formulate the generalnth-order Cartesian derivatives of internal coordinates, and to provide closed forms for general,nth-order transformation equations of anharmonic force fields, expressed as Taylor series, from internal to Cartesian or normal coordinate spaces.

ISSN:0026-8976    

DOI:10.1080/002689796173138

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The pairwise atom potential approach for calculating intermolecular potential energy surfaces is tested on the glyoxal–Ar complex with three isomeric structures Potential parameters of the Ar–H, Ar–C and Ar–O pairs were extracted from fittings to the experimentally determined H2–Ar and CO2–Ar potential surfaces It is found that when an anisotropic term reflecting the π electron distribution in the CO chromophore is added the pair potential calculation can generate three potential minima with reasonable energies and van der Waals stretching frequencies mimicking the three experimentally observed structures The electronically excited complex parameters extracted from anab initiocalculation done for H2CO(Ã)–Ar can predict the shift of the Ar position upon electronic excitation of glyoxal These observations indicate the transferability of properly determined pair potential parameters from one molecular system to another and the importance of anisotropy which reflects the electron density spatial distribution The effectiveness of the pairwise atom approach for performing expedient approximate intermolecular potential calculations is also evident although its usefulness for generating accurate potential surfaces has yet to be tested

ISSN:0026-8976    

DOI:10.1080/002689796173147

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Quantum mechanical calculations at the MP4/6-311G(2df,2pd)//MP2/6-31G(d,p) level of theory are reported for the compounds XBeO with X = NH3, NMe3, CO, N2, C2H2, C2H4, H2, H2CO and O2. The calculations show that BeO is a very strong Lewis acid. The X–BeO bond strength is betweenDe=69•5 kcal mo1-1for Me3NBeO andD3= 11•2 kcal mo1-1for π-bonded N2BeO. The calculated bond strength for the yet unknown donor–acceptor complex Me3NBeO is significantly higher than the strongest experimentally known main–group donor–acceptor complex Me3NA1C13(Do= 47•5 kcal mo1-1). Even the weak donor H2is bonded withDe= 18•5 kcal mo1-1. The compounds O2BeO and its isomer berylliumozonide BeO3should not be considered as donor–acceptor complexes. The results of the CDA method show that the donor–acceptor interactions in terms of orbital mixing are mainly described by the mixing of occupied orbitals of X with vacant orbitals of BeO, while the mixing of occupied orbitals of BeO with vacant orbitals of X is negligible. The topological analysis of the electronic charge distribution and the NBO partitioning scheme demonstrate that the X–BeO bonds have little or no covalent character; the bonds are caused by electrostatic attraction. The charge concentration at the donor atoms in the stronger bonded compounds is significantly deformed towards the beryllium atom.

ISSN:0026-8976    

DOI:10.1080/002689796173156

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The Nosé–Hoover chain thermostat of Martyna, Klein and Tuckerman has been adapted to work with our generalized valence bondab initiomolecular dynamics (AIMD) method, allowing us to perform AIMD in the canonical ensemble (constant number of particles temperature and volume). The canonical ensemble is ideal for simulated annealing which can be used to find global potential energy minima. The efficacy of constant-temperature-based AIMD simulated annealing is demonstrated for a Si6cluster and is compared with that of a non-constant-temperature-based method. It is found that in comparison with the velocity-scaling approach to annealing, constant-temperature based annealing gives better control over the annealing process and, at least in the case under study, results in attainment of the preferred geometry of the system more efficiently.

ISSN:0026-8976    

DOI:10.1080/002689796173165

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A simple octopolar fluid is considered whose particles interact via a square well (SW) potential plus point octopoles. The SW potential models the effects of quasi-spherical overlap and dispersion forces. An analytical equation for the free energy, which contains the explicit dependence on the potential parameters is derived by perturbation theory. The theory is used to analyse the influence of the octopole–octopole interaction on the thermodynamics. The octopolar effects on the vapour–liquid equilibrium are found to be much stronger than those of equivalent dipoles or quadrupoles. The theory is applicable to octopolar quasi spherical molecules and is used as a simple model for the vapour–liquid equilibrium of methane.

ISSN:0026-8976    

DOI:10.1080/002689796173174

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Vibrational frequency shifts have been calculated at the MP4(aug-cc-pVTZ) and MP4(cc-pVTZ)ab initiolevels for LiH, BH, HF, OH-, HDO, BF, CN-and HC1, in uniform electric fields of varying strength. Vibrational Stark tuning rates as well as strong-field frequency shifts have been computed. All eight molecules studied here display a similar variation of frequency with field: the curves are parabola shaped with negative curvatures. The molecules have different positions of ωmax, but in all cases ωmaxis located close to the field max max strength where ∂µ∥/∂r= 0.

ISSN:0026-8976    

DOI:10.1080/002689796173183

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Anharmonic potential energy surfaces for the coordinatesr(A–H) andr(A...B in the A–H...B H-bonds (H2O)2, HOHF-, and HOHOH-have been calculatedab initio. In addition, NMR nuclear shielding surfaces for the atoms A and B were calculated. The nuclear configuration interaction (CI) method was applied to achieve anharmonic vibrational states. These are the bases for the vibrationally averaged distancesr(A–H) andr(A–D) as well asr(A...B) (H) andr(A...B) (D), and dynamically corrected NMR chemical shifts. Changes of interatomic distances and chemical shifts upon deuteration are presented and discussed. The maximum dynamical corrections are 0•09 Å for the OH distance in HOHOH-and 9 ppm for δ(19F) in HOHF-. The maximum deuteration effects are 0•018 Å for the OH distance and 3 ppm for the δ(19F) NMR chemical shift in HOHF-.

ISSN:0026-8976    

DOI:10.1080/002689796173192

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The exchange repulsion formula proposed by Murrell and co-workers (Proc. Roy. Soc.(Lond.) 1965,A284, 566;J. chem. Phys.,1967, 47, 4916) is considered in detail. Potentially important terms missing in the formalism of Murrell and co-workers are identified and evaluated for the water dimer using several basis sets. Insights into the contributing terms are obtained by using localized molecular orbitals. The results point towards a relatively simple expression for intermolecular exchange repulsion, based on the isolated wavefunctions of the two overlapping species.

ISSN:0026-8976    

DOI:10.1080/002689796173200

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A comparative study of vibrational predissociation dynamics is presented. Two collinear models of the van der Waals complex are used with a realistic medium-strength coupling parameter. The predissociation rates are calculated by four different approaches: an accurate quantum mechanical method by the complex-scaling technique, first-order approximations in the diabatic (FOD) and adiabatic (FOA) basis, and purely classically. It is shown that FOA within the improved semiclassical Landau method provides an excellent description of the dynamical tunnelling of the system from all the quasibound states into continuum; at the same time, FOD yields noticeably higher rates though the transition probabilities are very low. At low excitation energies of the van der Waals bond, the classical description yields zero rates in accord with the KAM theorem. At higher excitation energies, the classical rates are higher than the quasiclassical rates since the classical system dissociates via the diffusion through the holes in the phase space which are still too narrow to let the quantum system escape. A simple explanation of a parallelism between quantum and classical rates is suggested under a condition when the first-order quantum treatment is applicable.

ISSN:0026-8976    

DOI:10.1080/002689796173219

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The adsorption site forp-xylene in ferrierite has been located using both powder diffraction (neutron and synchrotron X-ray) and energy minimization simulations. The latter also have been able to locate the low energy sites for the bridging hydroxyl acid sites (OD) in the framework. Evidence is found for appreciable structural changes which occur around the sorbed molecule.

ISSN:0026-8976    

DOI:10.1080/002689796173228

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor