  The transport phenomena of Lennard-Jones molecules through a structural
bottleneck driven by an external force are investigated by molecular dynamics
simulations. We observe two distinct molecular flow regimes distinguished by a
critical external force $F_{c}$ and find scaling behaviors between external
forces and flow rates. Below the threshold $F_{c}$, molecules are essentially
stuck in the bottleneck due to the attractive interaction between the
molecules, while above $F_{c}$, molecules can smoothly move in the pipe. A
critical flow rate $q_{c}$ corresponding to $F_{c}$ satisfies a simple
relationship with angles and the value of $q_{c}$ can be estimated by a simple
argument. We further clarify the role of the temperature dependence in the
molecular flows through the bottleneck.
