(384g) Metadynamics Simulations of Dissolution of Minerals. Overcoming Hidden Energy Barriers Problem

Free energy profiles of the dissolution of silica from several flat crystallographic surfaces of α-quartz were obtained using a novel scheme based on metadynamics [1]. The multidimensional free energy profiles thus obtained are analyzed and compared to other experimental and simulation data. They suggest that the reaction coordinate of single collective variable is not sufficient to accurately sample activation energies between the intermediate dissolution states of the silica unit. The true activation barriers are hidden by the adjacent free-energy basins during the averaging/integrating over the excess collective variables. Consequently, the resulting one-dimensional free-energy profiles underestimate the height of the energy barriers. A space of three collective variables is demonstrated to be sufficient to reconstruct, with help of metadynamics, the free-energy profiles of silica unit dissolution and to more accurately predict free-energy barriers between intermediate states. We believe that the activation energies obtained from the multidimensional free energy profiles calculated using the proposed methodology should be more correct when compared to the other conventional approaches based on the one-dimensional blue moon ensemble and umbrella sampling methods. The metadynamics calculations were perfomed using both a classical reactive force field ReaxFF [2] and ab initio methods [3].
[1]  A. Laio, F. L. Gervasio. Reports on Progress in Physics 71,  126601 (2008).
[2]  J. C. Fogarty, H. M. Aktulga, A. Y. Grama,  A. C. Van Duin,  S. A. Pandit,   The Journal of chemical physics 132, 174704 (2010)
[3]  P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli et al.  Journal of Physics: Condensed Matter 21, 395502 (2009).