Energy, force, and stiffness under large loads

Nanomehanical engineering and conventional chemistry place different demands on potential energy functions and emphasize different properties. Some of these differences are compared for the accuracies demanded by solution phase and machine phase chemistry. Other diffences result from the nanomechanical emphasis on force as a controllable parameter and on stiffness as a determinant of positioning errors in conventional chemistry, stiffness is of interest chiefly as a determinant of vibrational frequencies in spectroscopy, and force is rarely mentioned. Further, many nanomechanical systems apply large forces to bonds and to nonbonded interfaces. Although strained organic molecules can experience large bonded and onobonded forces, potential functions developed for chemistry must be examined for suitability before applying them to problems of nanomechanical design involving large loads.

Potential for chemical reactions

Because molecular mechanics methods are based on the notion of structures with well defined bonds, they cannot describe transformations that make or break bonds, and cannot predict chemical instabilities. Potential energy functions describing chemical reactions have been separately studied. Techniques that combine molecular mechanics potentials for describing large structures with reaction potentials or with quantum mechanical methods applied to small regions are useful in describing nano-mechanisms that make and break bonds. Bond cleavage and formation present computational challenges for molecular orbital methods. Accurate calculations often require extensive use of CI methods to account for electron correlation effects, raising the cost of computations and preventing computations are feasible at many points on the PES subsequent analytical studies often demand that the surface be described by some function fitted to those points. Accordingly, studies of molecular reaction dynamics usually rely on approximate potential energy surfaces. These are described either by fitting complicated functions to quantum mechanical calculations, or by adjusting a few parameters in a fixed functional form to make calculated reaction rates and their temperature dependence, fit experimental data.


Continuum representations of surfaces

nanomechanical components often are subject to forces dominated by bonding and overlap repulsion, but van der Waals attractions(London dispersion forces) between nanometer scale objects can also be substantial. The long range nature of thee forces motivates the use of continuum approximations. At small spacings, surface-surface interactions require a model that separately accounts for the first atomic layer, including overlap forces.