RESEARCH
1. Reaction kinetics and dissolution mechanisms of minerals
One of the most common geochemical processes that occurs on earth's surface is the dissolution of rocks and minerals by water.The global water cycle, environmental pollution, and acid rain are some of the factors that contribute towards accelerating the mineral-water dissolution process. Among all the minerals, silicates comprise the largest group of minerals, and as a result, they attract considerable interest from geochemists. In addition, silicates have multiple technological applications in the chemical industry including semiconductors, microelectronics, zeolite synthesis, glass, adhesives, and paints. All these applications have led to numerous experimental and computational studies focused on understanding the interactions at the silica-water interface. The bulk of silica is composed of Si-O-Si linkages, but the surface terminations consist of hydrophilic hydroxyl groups. The silica surfaces interact with water in two competing processes of physical adsorption and chemical dissolution. In the physical process, the hydroxyl groups adsorb water molecules by hydrogen bonding sometimes forming stable tessellated patterns, however, in the chemical process water interacts with the surface to cleave the Si-O-Si linkages resulting in hydrolyzed products. In both processes the role of surface hydroxyl groups is significant, and there have been experimental studies to detect these groups. The nature of the surface hydroxyl groups varies depending upon the porosity, particle size, crystal faces, and the pH. The change in pH alters the protonation state of the surface groups, significantly affecting the dissolution mechanism and the reaction rate.
2. Topological features of mineral surfaces
The most commonly investigated polymorphs of silica are alpha,(beta)-quartz and cristobalite because of their comparable refractive indices and density to amorphous silicates for which experimental data are available. So far, computational studies of surfaces have concentrated on a small set of crystallographic planes, such as the (100), (111), and (011) planes of ,(alpha)-quartz and cristobalite, and the (0001) plane of quartz. Both quartz and cristobalite have no preferred crystallographic cleavage plane, and therefore powdered or crushed samples of these silicates will have a complex structure, presumably composed of some combination of low index Miller planes. The motivation of the present work is to use efficient computational techniques to study and analyze a large set of surfaces and investigate surface topologies, so that a more comprehensive picture of the surface of crushed beta-cristobalite grains can be achieved. On the basis of the topology, it is also interesting to investigate the role of H-bonding between the surface hydroxyl groups and adsorbed water overlayers. In the present work, the MD method was chosen to obviate the computational expense of ab initio methodology, and here we also compare the results from two disparate potential energy functions. These potential energy functions are a key component of MD simulations and are used to describe molecular motion for the chemical system under study.
3. Photochemistry of reactions involving conical intersections and avoided crossings
We used multiconfiguration quasidegenerate perturbation theory and the fourfold-way direct diabatization scheme to calculate ab initio potential-energy surfaces at 3600 nuclear geometries of NH3. The calculations yield the adiabatic and diabatic potential-energy surfaces for the ground and first electronically excited singlet states and also the diabatic coupling surfaces. The diabatic surfaces and coupling were fitted analytically to functional forms to obtain a permutationally invariant 2x2 diabatic potential-energy matrix. An analytic representation of the adiabatic potential-energy surfaces is then obtained by diagonalizing the diabatic potential-energy matrix. The analytic representation of the surfaces gives an analytic representation of the four-dimensional conical intersection seam which is discussed in detail.
4. Non-adiabatic reaction dynamics
Electronically nonadiabatic or non-Born-Oppenheimer (non-BO) chemical processes (photodissociation, charge-transfer, etc.) involve a nonradiative change in the electronic state of the system. Molecular dynamics simulations typically treat nuclei as moving classically on a single adiabatic potential energy surface, and these techniques are not immediately generalizable to non-BO systems due to the inherently quantum mechanical nature of electronic transitions. Here we generalize the concept of a single-surface molecular dynamics trajectory to that of a coupled-surface non-BO trajectory that evolves "semiclassically" under the influence of two or more electronic states and their couplings. Five non-BO trajectory methods are discussed. Next, we summarize the results of a series of systematic studies using a database of accurate quantum mechanical reaction probabilities and internal energy distributions for several six-dimensional model bimolecular scattering collisions. The test set includes three kinds of prototypical nonadiabatic interactions: conical intersections, avoided crossings, and regions of weak coupling. We show that the coherent switching with decay of mixing (CSDM) non-BO trajectory method provides a robust and accurate way to extend molecular dynamics to treat electronically nonadiabatic chemistry for all three kinds of nonadiabatic interactions, and we recommend it for molecular dynamics simulations involving nonradiative electronic state changes.
5. Rare-event sampling algorithm- Army Ants Alogorithm
The most widely used algorithm for Monte Carlo sampling of electronic transitions in trajectory surface hopping (TSH) calculations is the so-called anteater algorithm, which is inefficient for sampling low-probability nonadiabatic events. We present a new sampling scheme (called the army ants algorithm) for carrying out TSH calculations that is applicable to systems with any strength of coupling. The army ants algorithm is a form of rare event sampling whose efficiency is controlled by an input parameter. By choosing a suitable value of the input parameter the army ants algorithm can be reduced to the anteater algorithm (which is efficient for strongly coupled cases), and by optimizing the parameter the army ants algorithm may be efficiently applied to systems with low-probability events. To demonstrate the efficiency of the army ants algorithm, we performed atom-diatom scattering calculations on a model system involving weakly coupled electronic states. Fully converged quantum mechanical calculations were performed, and the probabilities for nonadiabatic reaction and nonreactive deexcitation (quenching) were found to be on the order of 10(-8). For such low-probability events the anteater sampling scheme requires a large number of trajectories (similar to10(10)) to obtain good statistics and converged semiclassical results. In contrast by using the new army ants algorithm converged results were obtained by running 10(5) trajectories. Furthermore, the results were found to be in excellent agreement with the quantum mechanical results. Sampling errors were estimated using the bootstrap method, which is validated for use with the army ants algorithm.
PUBLICATIONS
13. Role of intrasurface hydrogen bonding on silica dissolution, S. Nangia and B. J. Garrison, J. Phys. Chem. C (accepted).
12. Advanced Monte Carlo approach to study evolution of quartz surface during the dissolution process, S. Nangia and B. J. Garrison, J. Am. Chem. Soc. 131, 9538-9546 (2009).
11. Ab-initio study of dissolution and precipitation reaction from edge, kink, and surface sites of quartz as a function of pH, S. Nangia and B. J. Garrison, Mol. Phys. 107 , 831-843 (2009) .
10. Ab Initio investigation of dissolution mechanisms in aluminosilicate minerals, C. P. Morrow, S. Nangia and B. J. Garrison, J. Phys. Chem. A 113, 1343-1352 (2009).
9. Reaction rates and dissolution mechanisms of quartz as a function of pH, S. Nangia and B. J. Garrison, J. Phys. Chem. A 112, 2027-2033 (2008).
8. Study of a family of 40 hydroxylated beta-cristobalite surfaces using empirical potential energy functions, S. Nangia, N. M. Washton, K. T. Mueller, J. D. Kubicki, and B. J. Garrison, J. Phys. Chem. C 111, 5169-5177 (2007).
7. Direct calculation of coupled diabatic potential-energy surfaces for ammonia and mapping of a four-dimensional conical intersection seam, S. Nangia and D. G. Truhlar, J. Chem. Phys. 124, 124309-13 (2006).
6. Non-Born-Oppenheimer molecular dynamics, A. W. Jasper, S. Nangia, CY. Zhu, and D. G. Truhlar, Acc. Chem. Res. 39, 101-108 (2006).
5. A new form of MgTa2O6 obtained by the molten salt method, A. K. Ganguly, S. Nangia, M. Thirumal, and P. L. Gai, J. Chem. Sci. 118, 37-42 (2006).
4. Can a single-reference approach provide a balanced description of ground and excited states? A comparison of the completely renormalized equation-of-motion coupled-cluster method with multireference quasidegenerate perturbation theory near a conical intersection and along a photodissociation coordinate in ammonia, S. Nangia and D. G. Truhlar, M. J. McGuire, and P. Piecuch, J. Phys. Chem A 109, 11643-11646 (2005).
3. Introductory lecture: Nonadiabatic effects in chemical dynamics, A. W. Jasper, CY. Zhu, S. Nangia, and D. G. Truhlar, Faraday Discus. 127, 1-22 (2004).
2. Coherent switching with decay of mixing: An improved treatment of electronic coherence for non-Born-Oppenheimer trajectories, CY. Zhu, S. Nangia, A. W. Jasper, and D. G. Truhlar, J. Chem. Phys. 121, 7658-7670 (2004).
1. Army ants algorithm for rare event sampling of delocalized nonadiabatic transitions by trajectory surface hopping and the estimation of sampling errors by the bootstrap method, S. Nangia, A. W. Jasper, T. F. Miller III, and D. G. Truhlar, J. Chem. Phys. 120, 3586-3597 (2004).