Understanding the complex behavior of particles at surfaces requires detailed knowledge of both macroscopic and microscopic processes that take place; also certain phenomena depend critically on temperature and gas pressure. To link these processes we combine state-of-the-art microscopic, and macroscopic phenomenological, theories. We apply our theory to the O͞Ru(0001) system and calculate thermal desorption spectra, heat of adsorption, and the surface phase diagram. The agreement with experiment provides validity for our approach which thus identifies the way for a predictive simulation of surface thermodynamics and kinetics. PACS numbers: 68.45.Da, 82.65.Dp, 82.65.My The study of the physical and chemical processes that take place at gas-surface interfaces have long been an area of intense research. This interest is both fundamental as well as driven by the possible discovery of important technological applications, e.g., in the field of heterogeneous catalysis, corrosion, etc. [1,2]. With respect to the field of the theory of adsorption of gases on solid surfaces, advancement in recent years has developed in two distinct, albeit complimentary, directions: (i) electronic structure calculations, at best done by density-functional theory (DFT), to determine the geometries, energetics, and vibrational properties of adsorbate covered surfaces, and (ii) phenomenological models, both for the thermodynamics and the kinetics [3] of the adsorbate. If one can assume that the geometry of the solid surface does not change dramatically and that adsorption occurs at well defined sites, one frequently employs a lattice gas model. A number of parameters enter this type of model, such as the binding energies and vibrational frequencies of a single adparticle in the various adsorption sites, and their mutual lateral interactions with adparticles in close-by sites. Traditionally, these parameters are adjusted in the theory in order to fit a variety of experimental data such as phase diagrams, heats of adsorption, infrared spectra, and thermal desorption data, etc. Such an approach, while useful, is clearly not necessarily predictive in nature, nor the parameters unique, and may not capture the physics of the microscopic processes that are behind the "best-fit" adjusted "effective" parameters.In this Letter, with the aim to improve upon this approach, we combine state-of-the-art procedures of (i) microscopic theories, i.e., DFT electronic structure calculations and (ii) macroscopic phenomenological approaches, i.e., lattice gas and rate equations, and Monte Carlo schemes. On doing this, we present a consistent first-principles-based approach for calculation of the thermodynamic and kinetic properties of an adsorbate, such as heats of adsorption, temperature programmed desorption (TPD) spectra, and the surface phase diagram. We have chosen the system of oxygen at Ru(0001) for which detailed structural [4-9], thermodynamic [10], and kinetic data [11,12] exist. We will show that, with the present approach, a realistic description...

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We formulate the proper statistical mechanics to describe the stretching of a macromolecule under a force provided by the cantilever of an Atomic Force Microscope. In the limit of a soft cantilever, the generalized ensemble of the coupled molecule-cantilever system reduces to the Gibbs ensemble for an isolated molecule subject to a constant force in which the extension is fluctuating. For a stiff cantilever, one obtains the Helmholtz ensemble for an isolated molecule held at a fixed extension with the force fluctuating. Numerical examples and predictions for experiments with cantilevers of differing stiffness are given for short and long chains of poly (ethylene glycol), based on parameter-free ab initio calculations.

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