  We introduce an efficient and robust method to compute alchemical free energy
differences, resulting from the application of multiple walker Adaptive Biasing
Force (ABF) in conjunction with strongly damped Langevin $\lambda$-dynamics.
Unbiased alchemical free energy surfaces are naturally recovered by
Thermodynamic Integration (TI). No manual optimization of the $\lambda$
schedule is required as the sampling of the $\lambda$ variable is continuous
and converges towards a uniform distribution. Free diffusion of $\lambda$
improves orthogonal relaxation compared to fixed $\lambda$ methods such as
standard TI or Free Energy Perturbation (FEP). Furthermore, the multiple walker
strategy provides coverage of orthogonal space in a generic way with minimal
user input and negligible computational overhead. Of practical importance, no
adiabatic decoupling between the alchemical and Cartesian degrees of freedom is
assumed, ensuring unbiased estimates for a wide envelope of numerical
parameters. We present two high-performance implementations of the method in
production molecular dynamics engines, namely NAMD and Tinker-HP, through
coupling with the Colvars open source library. These interfaces enable the
combination of the rich feature sets of those packages. We demonstrate the
correctness and efficiency of the approach on several real-world cases: from
solvation free energies up to ligand-receptor binding (using a recently
proposed binding restraint scheme) with both fixed-charge and polarizable
models. We find that, for a chosen accuracy, the computational cost is strongly
reduced compared to state-of-the-art fixed-lambda methods and that results
within 1~kcal/mol of experimental value are recovered for the most complex
system. The implementation is publicly available and readily usable by
practitioners of current alchemical methods.
