  We present an optimal detrended fluctuation analysis (DFA) and applied it to
evaluate the local roughness exponent in non-equilibrium surface growth models
with mounded morphology. Our method consists in analyzing the height
fluctuations computing the shortest distance of each point of the profile to a
detrending curved that fits the surface within the investigated interval. We
compare the optimal DFA (ODFA) with both the standard DFA and nondetrended
analysis. We validate the ODFA method considering a one-dimensional model in
the Kardar-Parisi-Zhang universality class starting from a mounded initial
condition. We applied the methods to the Clarke-Vvdensky (CV) model in $2+1$
dimensions with thermally activated surface diffusion and absence of step
barriers. It is expected that this model belongs to the nonlinear Molecular
Beam Epitaxy (nMBE) universality class. However, an explicit observation of the
roughness exponent in agreement with the nMBE class was still missing. The
effective roughness exponent obtained with ODFA agrees with the value expected
for nMBE class whereas using the other methods it does not. We also
characterized the transient anomalous scaling of the CV model and obtained that
the corresponding exponent is in agreement with the value reported for other
nMBE models with weaker corrections to the scaling.
