Renormalization Group Optimized Perturbation: αs from fπ

A new version of the optimized perturbation (OPT), implementing renormalization group properties consistently, allows to calculate the nonperturbative ratio F-pi/Lambda(MS) over bar of the pion decay constant and the basic QCD scale in the (MS) over bar scheme. From the experimental F-pi input value it gives a new determination of Lambda((MS) over bar) for n(f) = 2 and n(f) = 3, and thus of the QCD coupling constant (alpha) over bar (S) at various scales once combined with a standard perturbative evolution. The empirical convergence properties of the RGOPT modified series is exhibited up to the third order. We examine the various sources of theoretical uncertainties and obtain (alpha) over bar (S) (m(Z)) = 0.1174(-.0005)(+.0010) +/- .001 +/- .0005(evol), where the first evo., errors are estimates of the intrinsic theoretical uncertainties of our method, and the second errors come from present uncertainties in F-pi/F-0, where F-0 is F-pi in the exact chiral SU(3) limit.