The York Lab, in a series of three recently published papers, have made breakthroughs in the development, validation, and establishment of best practices for GPU-accelerated free energy simulation methods in AMBER. 

For details see:

Validation of Free Energy Methods in AMBER

Improved Alchemical Free Energy Calculations with Optimized Smoothstep Softcore Potentials

Alchemical Binding Free Energy Calculations in AMBER20: Advances and Best Practices for Drug Discovery