Beyond Lateral Pressure Profiles: Local Stress and the Traction Vector in MD Simulations

Abstract

Lateral pressure or stress profiles are routinely used to understand the mechanical behavior of lipid membranes from molecular dynamics (MD) simulations. However, the 1-dimensional stress profiles are not adequate to understand the multidimensional mechanical state of complex systems where there are membrane-protein and protein-protein interactions. Furthermore, the fact that the microscopic stress from MD simulations is not uniquely defined is a theoretical consideration that is most often ignored, which has acute practical consequences when atomistic models are considered. I will present our recent work (Phys. Rev. Lett. 114, 258102, 2015 and J. Chem. Theory Comput. 10, 691, 2014) on the development of objective 3D local stress calculations and its applications to lipid bilayers, mechanosensitive channels, and fibrous proteins. I will show how popular definitions of the microscopic stress violate mechanical equilibrium and present an unambiguous and physically sound definition based on the central decomposition of forces from multi-body potentials. I will also demonstrate the use of the traction vector as a powerful tool to visualize the local stress tensor on interfacial surfaces (such as the surface of a protein embedded in a membrane).