Ab initio metadynamics determination of temperature-dependent free-energy landscape in ultrasmall silver clusters

Abstract

ABSTRACT: This thesis concerns the study of the influence of temperature and environmental effects in silver clusters at different temperatures. In particular, we reconstruct the free energy surface of Ag2, Ag5 and Ag6. For Ag5 and Ag6, we apply ab-initio Well-Tempered Metadynamics simulations at different temperatures. This is based on the Born-Oppenheimer Approximation and Density Functional Theory (DFT) to describe inter-atomic forces of the electronic distribution. Then, we evolve the system by adding an artificial bias potential to explore different regions of the configuration landscape and to estimate the free energy surface at 10, 100, and 300 K with the radius of gyration and coordination number as collective variables, finding errors, at most, in the order of tens of meV. Relative free-energy differences between the planar and non-planar isomers of both clusters decrease with temperature, in agreement with the previously proposed stabilization of non-planar isomers. Interestingly, we find that Ag6 is the smallest silver cluster where entropic effects at room temperature boost the non planar isomer probability to a significant value, making probable a mixture of isomers. This way, we obtain thermal effects over the probability of each state of the system. For Ag2, we reconstruct the free energy surface of the dissociation process of a silver dimer, considering a water solvent environment. Here, we use the ASE-PLUMED interface to apply Quantum Mechanics/Molecular Mechanics (QM/MM) with Well-Tempered Metadynamics, where the silver dimer is described with quantum mechanics and the water molecules are classical. We use the distance between the silver atoms as the collective variable. We show that the addition of water molecules in the simulation promotes the dissociation process, decreasing the free-energy barrier between the bounded and unbounded states. Unlike the vacuum model, in the solvent embedded case, we find that forming a dimer bond requires a barriercrossing. To perform these simulations, we develop an interface between the atomic simulation environment (ASE) and the PLUMED plug-in. This interface enables performing enhanced sampling techniques and molecular dynamics analysis using quantum and classical codes implemented in ASE. We show the details of this development and present the tests to prove the correct performance of the interface. This new ASE-PLUMED interface enables simulating nanosystem electronic properties at more realistic conditions. These methods can help better describe larger nanoclusters and can be improved considering interactions with other environments.