Modified Morse potentials for classical molecular dynamics simulation of nuclear materials

<ns3:p>The objective of this paper is to demonstrate how Morse potentials can be modified to better approximate experimentally determined melting points of materials. Morse potentials are typically fit to, and in agreement with, zero temperature properties such as bond energy, equilibrium bond length, and force constant. However, high temperature properties are often very inaccurate. The gap this paper seeks to address is the lack of a systematic approach to development of simple interatomic potentials which can address both high temperature and low temperature material properties. In principle, these potentials should utilize physics informed analytical functional forms and have a small number of fitting parameters. Our approach for development of a modified Morse potential (MMP) is to connect three component Morse potentials (CMP) that differ by force constant via smoothing functions. The CMPs are responsible for different MMP regions which are defined by distance between atoms. The left-hand CMP is responsible for the repulsive part of the MMP, the unmodified Morse potential (UMP) is responsible for the bottom of the potential well, and the right-hand CMP is responsible for the attractive part of the MMP. The force constants of the left-hand CMP and right-hand CMP can be larger or smaller values than the UMP, which results in a stiffer or looser MMP at distances between atoms that would be sampled at high temperatures, far from the bottom of the potential well. This added flexibility allows the MMP to be tuned to the predicted melting point while preserving the zero temperature properties inherited from the original UMP. This work is motivated by the in-pile instrumentation program which has developed additively manufactured melt wires for measuring the temperature of operating nuclear reactors.</ns3:p>