  Recent results on supercomputers show that beyond 65K cores, the efficiency
of molecular dynamics simulations of interfacial systems decreases
significantly. In this paper, we introduce a dynamic cutoff method (DCM) for
interfacial systems of arbitrarily large size. The idea consists in adopting a
cutoff-based method in which the cutoff is cho- sen on a particle-by-particle
basis, according to the distance from the interface. Computationally, the
challenge is shifted from the long-range solvers to the detection of the
interfaces and to the computation of the particle-interface distances. For
these tasks, we present linear-time algorithms that do not rely on global
communication patterns. As a result, the DCM algorithm is suited for large
systems of particles and mas- sively parallel computers. To demonstrate its
potential, we integrated DCM into the LAMMPS open-source molecular dynamics
package, and simulated large liquid/vapor systems on two supercomputers:
SuperMuc and JUQUEEN. In all cases, the accuracy of DCM is comparable to the
traditional particle-particle particle-mesh (PPPM) algorithm, while the
performance is considerably superior for large numbers of particles. For
JUQUEEN, we provide timings for simulations running on the full system (458,
752 cores), and show nearly perfect strong and weak scaling.
