Dispersion Corrections at Planar Surfaces

When simulating a molecular system, a cutoff distance for interactions is often used to speed up the simulations. This is made at the cost of neglecting some interactions which will lead to inaccurate results for energy, pressure components and surface tension (for systems with surfaces). To compensate for the neglected long-range interactions, continuum corrections can be added to the surface tension, system energies and pressures. For a homogenous isotropic system this is straight-forward but for a system with a surface it is more complicated. In this work we have derived expressions for the corrections to the surface tension, system energies and pressures that are more general than previous results. When these corrections are added to multi-component systems with a surface (or single-component systems with vacuum) they compensate for the change in surface tension, system energy and pressures due to the finite cutoff. When simulating systems with no Coulomb-interactions, the structure of the system may change significantly if the cutoffs are too short. If this is the case then these corrections alone will not be enough. The solution is to add corrections to the force acting on each molecule added during the simulation, which we derive in this work. This solves the structural problem at low cutoffs and makes it possible to calculate an accurate surface tension independent of cutoff.