Reischl Bernhard, Watkins Matthew, Foster Adam S
Tampere University of Technology, Department of Physics, P.O. Box 692, FI-33101 Tampere, Finland.
COMP Centre of Excellence, Aalto University School of Science, Department of Applied Physics, P.O. Box 11100, FI-00076 Aalto, Finland.
J Chem Theory Comput. 2013 Jan 8;9(1):600-8. doi: 10.1021/ct3008342. Epub 2012 Dec 18.
High resolution atomic force microscopy (AFM) in liquids offers atomic scale insight into the structure at water/solid interfaces and is perhaps the only tool capable of resolving the nature of formed hydration layers. However, convolution between the imaging signal and the tip/surface interactions and hydration layers means that interpretation is far from straightforward. Modeling the complex imaging mechanism of atomic force microscopy in liquids requires calculation of the free energy profile as a function of the distance between AFM tip and surface. Its derivative is the best approximation for the force acting on the AFM tip, including entropic contributions from interactions with water molecules in hydration layers over the surface and around the tip apex. In order to establish a reliable approach for these simulations, we compare two methods of calculating free energy profiles from atomistic molecular dynamics simulations, umbrella sampling and free energy perturbation, on two model surfaces, calcium fluoride and calcium carbonate. Our results demonstrate that both methods effectively provide equivalent free energy profiles but offer different possibilities in terms of efficiency, constraints, and analysis of the free energy components.
液体环境中的高分辨率原子力显微镜（AFM）能够在原子尺度上洞察水/固界面的结构，或许是唯一能够解析所形成水化层本质的工具。然而，成像信号与探针/表面相互作用以及水化层之间的卷积效应意味着解读并非易事。对液体环境中原子力显微镜复杂成像机制进行建模，需要计算自由能分布随AFM探针与表面之间距离的变化。其导数是作用于AFM探针上力的最佳近似值，包括与表面及探针顶端周围水化层中水分子相互作用的熵贡献。为了建立用于这些模拟的可靠方法，我们在氟化钙和碳酸钙这两种模型表面上，比较了两种从原子分子动力学模拟计算自由能分布的方法，即伞形采样和自由能微扰法。我们的结果表明，这两种方法都能有效地提供等效的自由能分布，但在效率、约束条件以及自由能分量分析方面提供了不同的可能性。
