Radu Iftimie

Using a classical potential as an efficient importance function for sampling from an ab initio potential

In this paper the ab initio potential of mean force for the formic acid–water system is calculated in a Monte Carlo simulation using a classical fluctuating charge molecular mechanics potential to guide Monte Carlo updates. The ab initio energies in the simulation are calculated using density-functional theory (DFT) methods recently developed by Salahub et al. [J. Chem. Phys. 107, 6770 (1997)] to describe hydrogen-bonded systems. Importance sampling methods are used to investigate structural changes and it is demonstrated that using a molecular mechanics importance function can improve the efficiency of a DFT simulation by several orders of magnitude. Monte Carlo simulation of the system in a canonical ensemble at T=300 K reveals two chemical processes at intermediate time scales: The rotation of the H2O bonded to HCOOH, which takes place on a time scale of 3 ps, and the dissociation of the complex which occurs in 24 ps. It is shown that these are the only important structural “reactions” in the formic ac...

Spectral signatures and molecular origin of acid dissociation intermediates.

The existence of a broad, mid-infrared absorption ranging from 1000 to 3000 cm(-1) is usually interpreted as a signature for the existence of protonated water networks. Herein, we use cryogenic mixtures of water and hydrogen fluoride (HF) and show experimental and computational evidence that similarly wide absorptions can be generated by a broad distribution of proton-shared and ion pair complexes. In the present case, we demonstrate that the broadening is mainly inhomogeneous, reflecting the fact that the topology of the first solvation shell determines the local degree of ionization and the shared-proton asymmetric stretching frequency within H2O x HF complexes. The extreme sensitivity of the proton transfer potential energy hypersurface to local hydrogen bonding topologies modulates its vibrational frequency from 2800 down to approximately 1300 cm(-1), the latter value being characteristic of solvation geometries that yield similar condensed-phase proton affinities for H2O and fluoride. By linking the local degree of ionization to the solvation pattern, we are able to propose a mechanism of ionization for HF in aqueous solutions and to explain some of their unusual properties at large concentrations. However, an important conclusion of broad scientific interest is our prediction that spectral signatures that are normally attributed to protonated water networks could also reveal the presence of strong hydrogen bonds between un-ionized acids and water molecules, with important consequences to spectroscopic investigations of biologically relevant proton channels and pumps.

Efficient ab initio sampling methods in rate constant calculations for proton-transfer reactions

In this article, the classical potential based importance Monte Carlo sampling method of Iftimie et al. [J. Chem. Phys. 113, 4852, (2000)] is applied to an ab initio simulation of the proton transfer tautomerization reaction of malonaldehyde in an aprotic, nonpolar solvent. It is demonstrated that ad hoc bond-energy bond-order relations derived from bond evolution theory combined with Pauling’s valence bond ideas can be used to construct a molecular mechanics guidance potential for the simulation of the proton transfer reaction which improves the statistics of the calculation by three orders of magnitude. The sampling method is extended to simulations in which quantum effects are treated using the imaginary time path-integral representation. A new algorithm based on multiple Markov chain theory is introduced by which it is possible to obtain very short integrated correlation lengths in calculations of quantum static correlation functions.

An efficient Monte Carlo method for calculating ab initio transition state theory reaction rates in solution

In this article, we propose an efficient method for sampling the relevant state space in condensed phase reactions. In the present method, the reaction is described by solving the electronic Schrodinger equation for the solute atoms in the presence of explicit solvent molecules. The sampling algorithm uses a molecular mechanics guiding potential in combination with simulated tempering ideas and allows thorough exploration of the solvent state space in the context of an ab initio calculation even when the dielectric relaxation time of the solvent is long. The method is applied to the study of the double-proton transfer reaction that takes place between a molecule of acetic acid and a molecule of methanol in tetrahydrofuran. It is demonstrated that calculations of rates of chemical transformations occurring in solvents of medium polarity can be performed with an increase in the cpu time of factors ranging from 4 to 15 with respect to gas-phase calculations.

Spin tunneling through an indirect barrier : Tight-binding calculations

Spin-dependent tunneling through an indirect-band-gap barrier like the

A computational study of ultrafast acid dissociation and acid-base neutralization reactions. II. The relationship between the coordination state of solvent molecules and concerted versus sequential acid dissociation

\mathrm{GaAs}∕\mathrm{AlAs}∕\mathrm{GaAs}

Reaction mechanism and isotope effects derived from centroid transition state theory in intramolecular proton transfer reactions

heterostructure along the

Combining ab initio quantum mechanics with a dipole-field model to describe acid dissociation reactions in water: First-principles free energy and entropy calculations

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Using electronic polarization from the internal continuum (EPIC) for intermolecular interactions.

direction is studied by the tight-binding method. The tunneling is characterized by the proportionality of the Dresselhaus Hamiltonians at the

Molecular polarizabilities in aqueous proton transfer reactions

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