Yank hangs when running minimization for RMSD restraint

[jeffcomer]

During minimization, Yank continues using 100% of the CPU, but doesn't write any output or log entries. For example, I replaced "type: Harmonic" with "type: RMSD" in the example "p-xylene-explicit.yaml".

2020-02-18 19:18:14,368: Replica 17/25: final energy -504151.127kT 2020-02-18 19:18:20,861: Replica 18/25: initial energy -478622.787kT 2020-02-18 19:18:20,861: Using FIRE: tolerance 1.0 kJ/(nm mol) max_iterations 1000 2020-02-18 19:18:27,876: Replica 18/25: final energy -505014.560kT 2020-02-18 19:18:34,405: Replica 19/25: initial energy -478621.486kT 2020-02-18 19:18:34,405: Using FIRE: tolerance 1.0 kJ/(nm mol) max_iterations 1000

The last few log entries are from two days ago, while yank continues to use 100% of the CPU. On the other, hand, the run will complete if I create already equilibrated structures and run with minimize: off.