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In this paper (paper I) and the following paper (paper II) [C. Bratschi, H. Huber, and D. J. Searles, J. Chem. Phys. 126, 164105 (2007)], a new molecular dynamics algorithm implementing the Gibbs ensemble will be presented and then on applied to the liquid-vapor coexistence curve for two ab initio CO2 potentials. In paper I, the Gibbs ensemble molecular dynamics algorithm using non-Hamiltonian molecular dynamics techniques is introduced. It is shown that states of the correct probability density function are sampled and the correct exchange probability is generated. The extended system Nosé-Hoover formalism is used to generate a constant temperature ensemble with equal pressures in the subsystems, combined with single particle transfers between the subsystems, over several time steps, to get equal chemical potentials. [ABSTRACT FROM AUTHOR]

The Gibbs ensemble molecular dynamics algorithm introduced in the preceding paper (paper I) [C. Bratschi and H. Huber, J. Chem. Phys. v126, 164104 (2007)] is applied to two recently published CO2 ab initio pair potentials, the Bock-Bich-Vogel and symmetry-adapted perturbation theory site-site potentials. The critical properties of these potentials are calculated for the first time. Critical values and points in the single and two-phase zones are compared with Monte Carlo results to demonstrate the accuracy of the molecular dynamics algorithm, and are compared with experiment to test the accuracy of the potentials. Pressure calculations in the liquid, gas, and supercritical states are carried out and are used to explain potential-related effects and systematic discrepancies. The best ab initio potential yields results in good agreement with experiment. [ABSTRACT FROM AUTHOR]

A dimer and two trimers of carbon dioxide are known experimentally and have also been studied computationally. Whereas the calculated and experimental structures are in excellent agreement for the dimer and in fair agreement for the nonplanar trimer of C2 symmetry, the cyclic planar trimer shows a significant discrepancy in the angle β which measures the deviation from D3h symmetry. All calculations yield for this angle values around 40° in contrast to the experimental angle of (33.8±0.5)°. The latter was obtained from the experimental shift of the asymmetric vibrational stretch line of the trimer relative to the monomer using the resonant dipole-dipole interaction model. This model corresponds to the leading term in the molecular transition multipole expansion (TMPE) of the electrostatic coupling contribution to vibrational shifts. In this paper we go beyond the resonant dipole-dipole interaction model and include higher-order moments. To this end we use the first-order perturbation approach in combination with the (CO2)2 ab initio symmetry-adapted perturbation theory (SAPT)-s potential to calculate the vibrational shifts and scale the CO2 SAPT-s point charges to reproduce the experimental transition dipole moment of the gas-phase CO2 asymmetric stretch. The transition dipole-dipole term in the TMPE of the electrostatic coupling plays a predominant role in the vibrational shifts of the CO2 dimer and nonplanar trimer, however, to explain the vibrational shifts of the cyclic planar trimer the inclusion of the higher-order vibrationally induced moments is needed. As the angle β in this trimer approaches the value of 40° the dipole-dipole term in the electrostatic coupling disappears and the sum of the higher-order terms becomes important for the correct description of the vibrational shifts. Hence applying the resonant dipole-dipole interaction model to the experimentally observed vibrational shift for the cyclic planar trimer gives the wrong angle β for this trimer. The higher-order terms in the TMPE of the electrostatic coupling which are not negligible for the vibrational shifts of the cyclic planar trimer include vibrationally induced dipole-octupole and quadrupole-quadrupole interactions. [ABSTRACT FROM AUTHOR]

Model calculations to investigate the deuteron quadrupolar relaxation in liquid water are performed. Techniques not amenable to experiment, such as switching on and off the intermolecular or intramolecular electric field gradients and simulating rigid liquid water, give insight into the microscopic effects leading to relaxation. In experimental studies it is usually assumed that the deuteron quadrupolar relaxation is governed largely by the reorientational motion of an average electric field gradient, and the error in this assumption is readily extracted from the model calculations. As expected, this error is significant for deuterons in hydrogen bonds. These model calculations should provide a guide to better understanding of quadrupolar relaxation and experimental evaluation of relaxation. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An approach is presented that allows NMR relaxation rates to be determined for a complex mixture, and it is applied to a dimethyl sulfoxide/water solution. The calculation of the spin-lattice relaxation times (T1) and relaxation rates (R1 = (T1)(super -1) is discussed.

Deuterium quadrupole coupling constants were obtained for 39 sites from ab initio SCF calculations. An accuracy comparable to the experimental one is reached with a moderately large basis set of very high local quality. Comparison with experiment shows that electron correlation does not contribute substantially to this property. The experimental values for DNCO and DFCO should be reexamined, the value for CD3Cl in the literature was not properly transformed to the bond axes system, and the experimental value for ND3 agrees with the calculated one, thus showing that a dynamical model is not necessary. Several empirical relations are presented, and a simple classical model is proposed which relates the deuterium quadrupole coupling constant to the bond length with high accuracy. A theoretical study of the HCOOH dimer gives some insight into the importance of different mechanisms which lower the quadrupole coupling constant in the solid. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed with a new ab initio argon–argon pair potential from the literature for pressures up to 1000 MPa and temperatures up to 600 K in the supercritical phase as well as for points in the liquid phase of argon. Many properties, such as the compression factor z, the pair distribution function g, the enthalpy H, the internal energy U, the molar heat capacities CV,m and CP,m, the speed of sound c, the adiabatic and isothermal compressibilities βS and βT, the thermal pressure coefficient γV, the self-diffusion coefficient D, the thermal conductivity λ, and the shear viscosity η are calculated and compared to experimental values. These comparisons together with comparisons from calculations of similar quality for condensed neon give hints of the influence of many-body interactions on different properties. Structural and transport properties, but also thermodynamical properties like heat capacities, compressibilities and sound velocities, are quite accurately described within this model even at high pressures and in the liquid phase. Therefore, the calculated values are expected to give reliable predictions in ranges where experimental data are not available and it may be concluded, that many-body interactions have little influence on these properties. However, pressure, internal energy and enthalpy, show increasing absolute deviations with higher density and are not well reproduced without the inclusion of many-body interactions. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thermodynamical and structural properties including the equation of state, the second virial coefficient, the enthalpy and internal energy, the molar heat capacity, the speed of sound, the thermal expansion and pressure coefficients, the compressibility, and the pair distribution function are calculated in an ab initio approach for supercritical and liquid neon. The neon dimer potential energy curve has been obtained previously from ab initio calculations and is applied in classical molecular dynamics simulations. Care was taken to eliminate all possible errors thus reducing the remaining error in the supercritical state at higher temperatures to two sources, namely, the inaccuracies in the quantum chemical potential curve and the two particle approximation in the simulation. At lower temperatures, there is in addition an error due to the classical simulation. The calculated properties will be used as benchmarks in future work to investigate the influence of an improved potential curve and of an inclusion of the three particle potential in the simulation. [ABSTRACT FROM AUTHOR]

The quadrupole coupling constant and asymmetry parameter for the deuteron in liquid heavy water was determined using purely theoretical methods. Molecular-dynamics simulations with the ab initio potential-energy surface of Lie and Clementi were used to generate snapshots of the liquid. The electric-field gradient at the deuteron was then calculated for these configurations and averaged to obtain the liquid quadrupole coupling constant. At 300 K a quadrupole coupling constant of 256±5 kHz and an asymmetry parameter of 0.164±0.003 were obtained. The temperature dependence of the quadrupole coupling constant was investigated. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed with ab initio calculated potentials and electric field gradient curves on different levels of approximation, to study their influence on the relaxation time and diffusion coefficient of fluid 21Ne. Additional properties, such as quadrupole couplings, effective electric field gradient correlation times, and diffusion correlation times, are studied to get a better understanding of the underlying mechanisms. Semiquantitative models for the total, self and cross correlation functions of the electric field gradient are developed. [ABSTRACT FROM AUTHOR]