摘要:

The two-dimensional melting behaviour of the inverse-twelfth-power system and its dependence on boundary and initial conditions has been re-examined with long molecular dynamics simulation runs. The system consists of 1225 particles, which in a first series of runs is being simulated under standard periodic boundary conditions. It is feasible to reconcile some central simulation results with the experimental data of Murrayet al., 1987,Phys. Rev. Lett.,58,1200 for possible two-stage melting of submicron-sized charged colloidal spheres in liquid water. In another series of simulations, the particles are confined to a roughly rectangular cell with irregularly shaped walls. The energy as a function of density is found to be very sensitive to the choice of boundary conditions. The dependence on the initial conditions is studied by setting up the latter simulations with different sets of starting coordinates—some of them probably those of a hexatic phase—which were directly obtained from Murrayet al.'s experimental data for colloidal suspensions. Using the short rangedr-12potential, molecular dynamics simulations of these configurations were examined over five million time steps each. The initial order, even if experimentally found to be consistent with that of a hexatic phase, is not seen to undergo any rapid breakup. This suggests that kinetic bottlenecks limiting equilibration are still important, even for runs of this length.

ISSN:0026-8976    

DOI:10.1080/00268979300102041

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Accurate calculations of the non-expanded dispersion energies and damping functions, associated with theR-nmultipolar dispersion energies fornˇ- 10, are carried out for the six pair interactions arising from ground state H, He and Li. Highly correlated monomer wavefunctions, for both the relevant ground and excited states, are employed in the computations and the results are used to discuss the relative scale of the damping functions with interatomic separation. The results for the damping functions are also used to help assess the validity of various choices, in the literature, for empirical scaling parameters that connect the damping functions for a given interaction to those for H(1s)-H(1s) as a standard interaction and a new, effective, scaling procedure for heteronuclear dimers is proposed. The discussion also contains a rationalization of the trends in the scaling parameters, as a function ofR, which is based on the extent to which the lowest S →L≠ 0 transitions contribute to the dispersion energy coefficients for a given interaction relative to the H-H interaction.

ISSN:0026-8976    

DOI:10.1080/00268979300102051

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The recently introduced continuum-configurational-bias method for Monte Carlo simulations is employed for the generation of large samples of many-chain n-alkane systems with chain lengths of 11, 24 and 71 carbon atoms. The simulations are used to investigate the adequacy of representing methylene groups as united-atom Lennard-Jones interaction sites, and to test the configurational-bias approach against traditional random moves and reptation moves with respect to the computational efficiency and numerical stability of the calculated ensemble averages. The results of simulations with constant pressure, temperature, and number of molecules demonstrate that, with an appropriate mixture of different types of Monte Carlo moves, an efficient and stable strategy can be obtained. Adjustment of the Lennard-Jones parameters leads to results that are in good agreement with experimental data for the density of liquid alkanes over a large temperature interval.

ISSN:0026-8976    

DOI:10.1080/00268979300102061

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The liquid structure and thermodynamic properties are investigated for simple models of associating fluids. All of the fluids considered have a Lennard-Jones (LJ) spherical core with various off-centre association sites. The second virial coefficient and bond length distribution are calculated. Wertheim's theory of associating fluids is extended to molecules with an LJ core by way of a perturbation expansion; the reference fluid potential contains the association sites plus a soft repulsive core. Predictions from the theory are compared with Monte Carlo computer simulation results. The liquid structure of the reference fluid is in good agreement with that of the fluid with the full potential (LJ plus association sites). In addition to the perturbation theory, a pseudomixture theory is developed in which a pure associating fluid is treated as a mixture of bonded and unbonded components. The pseudomixture theory, compared with perturbation theory, requires fewer computer simulation results for its implementation, and should be useful in developing a realistic theory for polyatomic associating fluids such as methanol and water.

ISSN:0026-8976    

DOI:10.1080/00268979300102071

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Monte Carlo simulation data are presented for the triplet correlation functiong(3)(r,s,t), for a hard sphere system. Results are reported for reduced densities ρσ3between 0·3 and 0·88, and reduced distancesr/σ from 1·02 to 2·42 for all possible configurations. Comparisons with the PY3 theory show excellent agreement for all cases studied. A table of the complete simulation results is available separately.

ISSN:0026-8976    

DOI:10.1080/00268979300102081

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report a molecular simulation study of methane adsorption and phase transitions on a graphite substrate. Grand canonical Monte Carlo simulations have been carried out to study the adsorption isotherm and heat of adsorption at 77·5 K, using a uniform 10–4–3 substrate potential. Good agreement with experiment is found. Several layering transitions are observed at this temperature. Freezing transitions in the monolayer are found in the simulations, and are in qualitative agreement with experiment. Canonical Monte Carlo simulations have been used to study the commensurate-incommensurate transition of methane on a graphite substrate at conditions near the complete monolayer at 40·0 K, using a periodically varying adsorbate-adsorbent potential. The results are in qualitative agreement with experiments. A new method has been proposed to make a more realistic choice of the dimensions of the simulation cell. The effect of varying the corrugation of the fluid-wall potential on the commensurate and incommensurate phases is explored. Several sets of potential parameters have been used in the literature, and we compare results obtained with these sets throughout our simulations.

ISSN:0026-8976    

DOI:10.1080/00268979300102091

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Extended-basis-setab initiocalculations have been carried out to obtain the hydrogen chemical shielding tensor for water as a function of the internal geometrical coordinates. The results of the calculations show that the isotropic shielding and the anisotropy are most dependent on the oxygen-hydrogen bond length, and that the variation with the bond length is nearly linear. The anisotropy shows a significantly greater dependence on stretching the OH bond of the other hydrogen than does the isotropic shielding. Furthermore, we find that the deformation of water's structure that occurs in ice Ih is associated with a decrease in anisotropy instead of the expected increase.

ISSN:0026-8976    

DOI:10.1080/00268979300102101

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Fractional condensation in seeded supersonic free jets of SF6has been determined at different locations along the jet axis and at different stagnation pressures using a diode laser absorption technique. Rotational temperature and relative number densities of SF6have been determined fromQ-branch absorption of thev3band. The fractional condensation increases with increased stagnation pressure as well as with increased dilution. The onset pressure for condensation of SF6in pure SF6jets is above 2000 torr, whereas in seeded jets, it is much lower (900–1200 torr). Calculated dimer mole fractions do not account for the observed degree of condensation. Experimentally observed local onset number density can be reasonably correlated with the trimer critical model of Fisher.

ISSN:0026-8976    

DOI:10.1080/00268979300102111

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An implementation of coupled Hartree-Fock theory for static quadrupolar perturbations is used to calculate the quadrupole polarizabilityCand mixed dipole-quadrupole hyperpolarizabilityBfor anions in crystals. The ions studied are F-in LiF and NaF, O2-in MgO, S2-in MgS, H-in LiH, and C4-in Be2C. In-crystal compression produces a more than fourfold reduction inC(F-) to 9·7 a.u. in LiF and 11·6 a.u. in NaF, and nominally infinite reductions for O2-and S2-. Low anisotropies ΔCindicate retention of approximate spherical symmetry for all but C4-. The hyperpolarizabilityBshows similar reductions. Implications for quadrupolar shell models of Cauchy violation are discussed.

ISSN:0026-8976    

DOI:10.1080/00268979300102121

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The multireference single- and double-excitation configuration interaction method (MRD-CI) is applied for the computation of the potential energy surfaces of each of the component electronic states in the A2Σ+-X2Π transition of the NCO radical. The ability of this theoretical approach to deal with more than one state of the same symmetry is advantageous in this study because both the A2Σ+and the X2Π states have2A′ components for bent molecular geometries. The calculations are found to be quite accurate (±0·01a0) in predicting equilibrium bond length changes which occur between the (linear) A2Σ+and X2Π states. Similarly good experience is noted for the corresponding changes in vibrational frequencies. TheT0value for the transition is computed to be 23 131 cm-1, in good agreement with the observed result of 22 754 cm-1. The A2Σ+lifetime is computed to be 267 ns, whereas observed values for several of its lowest vibronic levels range from 322 to 361 ns. Rotational coupling between the two electronic states is kept to a minimum by virtue of the fact that the corresponding potential surfaces do not approach each other more closely at non-equilibrium geometries than theT0value indicated above.

ISSN:0026-8976    

DOI:10.1080/00268979300102131

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor