Free Energy Approaches for Modeling Atomic Force Microscopy in Liquids.

Area of Science:

• Surface science
• Computational chemistry
• Nanotechnology

Background:

• Atomic Force Microscopy (AFM) provides atomic-scale insights into water/solid interfaces.
• Interpreting AFM data is complex due to tip/surface interactions and hydration layers.
• Accurate modeling of AFM imaging mechanisms in liquids is crucial.

Purpose of the Study:

• To compare two methods for calculating free energy profiles in AFM simulations: umbrella sampling and free energy perturbation.
• To establish a reliable simulation approach for understanding hydration layers at interfaces.
• To evaluate the efficiency and analytical capabilities of each method.

Main Methods:

• Atomistic molecular dynamics simulations were employed.
• Free energy profiles were calculated using umbrella sampling and free energy perturbation.
• Simulations were performed on model surfaces: calcium fluoride and calcium carbonate.

Main Results:

• Both umbrella sampling and free energy perturbation yielded equivalent free energy profiles.
• The methods differ in efficiency, constraint implementation, and analysis of free energy components.
• The derivative of the free energy profile approximates the force on the AFM tip, including hydration effects.

Conclusions:

• Both tested methods are effective for modeling AFM imaging in liquids.
• The choice of method depends on specific research needs regarding efficiency and analysis.
• Reliable simulation approaches are essential for interpreting AFM data at hydrated interfaces.