When studying fluids with molecular dynamics simulations, periodic boundaries are usually used to model the infinite bulk fluid surrounding the primary cell. For homogeneous systems this is, as a rule, the most appropriate way. For inhomogeneous systems, e.g. systems with a fluid-vapour interface, periodic boundaries have some disadvantages. Therefore, an alternative for periodic boundaries, called the shifted reflective boundary, is proposed for modelling such systems. From a computational point of view, this type of boundary is no more difficult to implement than periodic boundaries. It is shown that the shifted reflective boundary results in a stable spatial fluid-vapour configuration with one fluid-vapour interface, while retrieving the same numerical results for the thermodynamic properties, e.g. the surface tension, as molecular dynamics simulations with periodic boundaries. Molecular dynamics simulations with shifted reflective boundaries also need fewer particles than corresponding simulations with periodic boundaries.