Many theoretical simulations of atomic force microscopy (AFM) have been performed so far to clarify the underlying physics in the experimental AFM images which are intuitively difficult to understand what are observed in them. Most of the simulations are based on dynamics of the atoms constructing a tip, a substrate and samples. The first step of the general AFM simulation is to calculate the overall structure either which has a minimal potential energy or motion of which has been proceeded with for one MD period in the condition that some atoms are fixed. The second step is to sum up the forces which are felt by all the tip atoms to obtain the overall force which is expected to be measured in the corresponding experiment.
The accuracy of time and position of all the atoms is the feature of this methodology. However, this feature is the reason of the high computational cost, which is in proportional to the squared number of atoms. For the simulation of the system which is immersed in a liquid or a gas, the all atoms of such an environment should be incorporated into the model and the number of atoms of the system is much more than the models in vacuum. Additionally, to explain the effect of an environment, we have to take an average of some values which have the large dispersion in many cases and it will need large calculation steps.
To overcome this difficulty, we use the method of statistical mechanics, Reference Interaction Site Model (RISM), to model the environment system and combine it with the usual dynamical method for non-environment atoms. In this study, we simulate the image of non-contact AFM for a simple model of a tip and a substrate of graphite sheet in aqueous environment. We explain the general feature of the images obtained in liquids by showing our results of different parameters such as temperature, water density tip height and so on.
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