Erratum: “Going beyond the frozen core approximation: Development of coordinate-dependent pseudopotentials and application to Na 2+” [J. Chem. Phys. 138, 054110 (2013)]

Nonequilibrium Solvent Effects during Photodissociation in Liquids: Dynamical Energy Surfaces, Caging, and Chemical Identity

In the gas phase, potential energy surfaces can be used to provide insight into the details of photochemical reaction dynamics. In solution, however, it is unclear what potential energy surfaces, if any, can be used to describe even simple chemical reactions such as the photodissociation of a diatomic solute. In this paper, we use mixed quantum/classical (MQC) molecular dynamics (MD) to study the photodissociation of Na₂⁺ in both liquid Ar and liquid tetrahydrofuran (THF). We examine both the gas-phase potential surfaces and potentials of mean force (PMF), which assume that the solvent remains at equilibrium with the solute throughout the photodissociation process and show that neither resemble a nonequilibrium dynamical energy surface that is generated by taking the time integral of work. For the photodissociation of Na₂⁺ in liquid Ar, the dynamical energy surface shows clear signatures of solvent caging, and the degree of caging is directly related to the mass of the solvent atoms. For Na₂⁺ in liquid THF, local specific interactions between the solute and solvent lead to changes in chemical identity that create a kinetic trap that effectively prevents the molecule from dissociating. The results show that nonequilibrium effects play an important role even in simple solution-phase reactions, requiring the use of dynamical energy surface to understand such chemical events.

Comment on "Going beyond the frozen core approximation: development of coordinate-dependent pseudopotentials and application to Na2(+)" [J. Chem. Phys. 138, 054110 (2013)]

The new coordinate-dependent pseudopotential for Na2(+) by Kahros and Schwartz [J. Chem. Phys. 138, 054110 (2013)] is assessed and compared to the pseudopotential approach by Fuentealba et al. [Chem. Phys. Lett. 89, 418 (1982)] which incorporates the coordinate-dependent core-polarization potential by Müller and Meyer [J. Chem. Phys. 80, 3311 (1984)]. In contrast to the latter approach, the one by Kahros and Schwartz does not reproduce the accurately known experimental data and∕or high level theoretical results for Na2(+). The treatment of core polarization by Kahros and Schwartz neglects the dynamic polarization of atomic cores which is much more important for Na2(+) than the static one. On the other hand, the Kahros and Schwartz method heavily overestimates frozen-core corrections at the Hartree-Fock level by compounding them with artifacts of a superposition of non-norm-conserving pseudopotentials.

Response to "Comment on 'Going beyond the frozen core approximation: development of coordinate-dependent pseudopotentials and application to Na2(+)'" [J. Chem. Phys. 139, 147101 (2013)]

Stoll, Fuentealba, and Szentpály (SFS) argue that the coordinate-dependent pseudopotential we developed for the sodium dimer cation molecule is inferior to other potentials that have been presented in the literature for this molecule. The goal of our work, however, was to present a novel method for the development of rigorous coordinate-dependent pseudopotentials. Our method is designed to reproduce all-electron Hartree-Fock calculations without the inclusion of adjustable parameters. Moreover, our method starts from the superposition of unoptimized, non-norm-conserved atomic potentials, so that when complete, the resulting norm-conserving potential can reproduce an all-electron Hartree-Fock calculation without the inclusion of adjustable parameters. We chose the sodium dimer cation system as a proof of principle for our method, and showed that our method does indeed allow a one-electron calculation to correctly reproduce the all-electron Hartree-Fock calculation from bonding to the dissociation limit. Our purpose in developing this method is to use such potentials in condensed-phase mixed quantum/classical molecular dynamics simulations, where inclusion of valence polarization effects is unimportant or can be added on after the fact. Thus we do not claim that our method provides a potential that is superior to potentials that have been specifically constructed to go beyond the static exchange approximation and/or include valence polarization effects-such potentials are beyond the scope of our work. We also note that although we made a numerical error in the application of our method to Na2(+) in our original work [A. Kahros and B. J. Schwartz, J. Chem. Phys. 138, 054110 (2013)] that led to an overestimation of the magnitude of core polarization effects for this particular molecule, out method does work as derived for this molecule and the error does not affect the significance of our method or its general applicability.