Volume 133, Issue 11, 21 September 2010
Index of content:
: A Monte Carlo study with flexible polarizable classical interaction potentials" title="Aqueous solvation of : A Monte Carlo study with flexible polarizable classical interaction potentials" />
A theoretical study of the hydration of arsenious acid is presented. This study included ab initio calculations and Monte Carlo simulations. The model potentials used for the simulations were ab initio derived and they include polarizability, nonadditivity, and molecular relaxation. It is shown that with these refined potentials it is possible to reproduce the available experimental evidence and therefore permit the study of clusters, as well as of the hydration process in solution. From the study of stepwise hydration and the Monte Carlo simulation of the condensed phase it is concluded that presents a hydration scheme similar to an amphipathic molecule. This phenomenon is explained as due to the existence of both a positive electrostatic potential and a localized lone pair in the vicinity of As. These results are used to rationalize the known passage of through aqua-glyceroporines.
Communication: The role of the positivity -representability conditions in natural orbital functional theory133(2010); http://dx.doi.org/10.1063/1.3481578View Description Hide Description
The positivity conditions for the -representability of the reduced density matrices are considered to propose a new natural orbital functional. The Piris reconstruction functional, which is based on an explicit form of the two-particle cumulant is used to reconstruct the two-particle reduced density matrix. A new approach for matrix, satisfying rigorously , , and necessary conditions, leads to Piris Natural Orbital Functional 4 (PNOF4). The theory is applied to the dissociation of selected diatomic molecules. The equilibrium distances, dipole moments, harmonic frequencies, anharmonicity constants, and binding energies of the considered molecules are presented. The values we have obtained are very accurate results comparing with the experimental data.
133(2010); http://dx.doi.org/10.1063/1.3483462View Description Hide Description
The adiabatic ionizationenergy [in units of , and the dissociation energy of HD have been determined using a hybrid experimental-theoretical method. Experimentally, the wave numbers of the and transitions to singlet Rydberg states were measured by laser spectroscopy and used to validate predictions of the electron binding energies by multichannel quantum defect theory. Adding the transition energies, the electron binding energies and previously reported term energies of the state led to a determination of the adiabatic ionizationenergy of HD and of rovibrational energy spacings in . Combining these measurements with highly accurate theoretical values of the ionizationenergies of the one-electron systems H, D, and further enabled a new determination of the dissociation energy of HD.
Communication: Prediction of the rate constant of bimolecular hydrogen exchange in the water dimer using an ab initio potential energy surface133(2010); http://dx.doi.org/10.1063/1.3481579View Description Hide Description
We report the properties of two novel transition states of the bimolecular hydrogen exchange reaction in the water dimer, based on an ab initio water dimer potential [A. Shank et al., J. Chem. Phys.130, 144314 (2009)]. The realism of the two transition states is assessed by comparing structures, energies, and harmonic frequencies obtained from the potential energy surface and new high-level ab initio calculations. The rate constant for the exchange is obtained using conventional transition state theory with a tunneling correction. We employ a one-dimensional approach for the tunneling calculations using a relaxed potential from the full-dimensional potential in the imaginary-frequency normal mode of the saddle point, . The accuracy of this one-dimensional approach has been shown for the ground-statetunneling splittings for H and D-transfer in malonaldehyde and for the reaction [Y. Wang and J. M. Bowman, J. Chem. Phys.129, 121103 (2008)]. This approach is applied to calculate the rate constant for the exchange and also for . The local zero-point energy is also obtained using diffusion Monte Carlo calculations in the space of real-frequency-saddle-point normal modes, as a function of .
Communication: Tracing phase boundaries via molecular simulation: An alternative to the Gibbs–Duhem integration method133(2010); http://dx.doi.org/10.1063/1.3486090View Description Hide Description
Precise simulation of phase transitions is crucial for colloid/protein crystallization for which fluid-fluid demixing may be metastable against solidification. In the Gibbs–Duhem integration method, the two coexisting phases are simulated separately, usually at constant-pressure, and the phase boundary is established iteratively via numerical integration of the Clapeyron equation. In this work, it is shown that the phase boundary can also be reproduced in a way that avoids integration of Clapeyron equations. The two phases are simulated independently via tempering techniques and the simulation data are analyzed according to histogram reweighting. The main output of this analysis is the density of states which is used to calculate the free energies of both phases and to determine phase coexistence. This procedure is used to obtain the phase diagram of a square-well model with interaction range , where is the particle diameter. The phase boundaries can be estimated with the minimum number of simulations. In particular, very few simulations are required for the solid phase since its properties vary little with temperature.