Specific Reaction Path Hamiltonian for Proton Transfer in Water: Reparameterized Semiempirical Models

Abstract

The semiempirical MNDO-based AM1 and PM3 methods and the orthogonalization-corrected OM1, OM2, and OM3 models were reparameterized to improve their description of bulk water and of proton transfer in water. Reference data included the gas-phase geometries and energies of the water molecule, small water clusters, the hydronium ion, and small hydronium ion-water clusters, as well as the gas-phase potential energy surface for proton transfer between the two water molecules in a Zundel ion, all calculated at the MP2/aug-cc-pVTZ level of theory. Combined QM/MM molecular dynamics simulations were carried out for bulk water and for a proton solvated in water using large cluster models. Both the authentic and reparameterized semiempirical models were employed in the simulations. The reparameterization led to significantly better results in all cases. The new set of OM3 parameters gave the best overall results for the structural and dynamic properties of water and the hydrated proton, with a small but finite barrier of 0.1-0.2 kcal/mol in the potential of mean force for proton transfer, in agreement with ab initio path-integral molecular dynamics simulations. The reparameterized OM3 model is expected to be useful for efficient modeling of proton transfer in aqueous solution.

Explicit proton transfer in classical molecular dynamics simulations

“…However, these methods do not provide accurate results for proton transfer reactions in water, primarily because they all underestimate the binding energy between water molecules. As a consequence, the first shell coordination numbers of the hydrated proton are too high, and the Zundel complex is overstabilized . Therefore, reparameterization is essential, as was done by Wu et al for the MNDO and OM n methods, and Goyal et al for the SCC‐DFTB method .…”

“…Despite these limitations, ab initio simulations have provided important atomistic insights into the transfer mechanism of hydrated protons, which can be used for developing and parameterizing more approximate approaches. In an effort to gain efficiency, while preserving the detailed description of the excess proton, approaches based on semiempirical methods and hybrid quantum/classical protocols have been proposed …”

Infrared Spectra of Deprotonated Dicarboxylic Acids: IRMPD Spectroscopy and Empirical Valence‐Bond Modeling

“…To extend the size and time scales inherent to quantum approaches, semi‐empirical simulations have been performed to study proton transfer reactions in water clusters,, and QM/MM approaches have been used to model an excess proton in water and enzymatic activity ,. The need of a multistate description in such complex environments has led to extensions of the empirical valence‐bond (EVB) model pioneered by Warshel and co‐workers, notably to address proton transport in water ,.…”

Adaptive quantum/molecular mechanics: what have we learned, where are we, and where do we go from here?

“…One approach is to designate all water molecules that may enter the solvation shell as QM, but doing so significantly increases the size of the QM subsystem and the associated computational costs. Alternatively, one may prohibit solvent exchange during simulations, e.g., by imposing certain constraints or restraints to the boundary between the solvation shell and the bulk water . However, doing so substantially limits the scope of the simulations, because many properties, such as diffusion coefficients, will not be properly calculated.…”