The formula given in de Haan and Ferreira, 2007 (Springer). Note that the latter differs from that found in Drees, Ferreira and de Haan.
gpd.abias(shape, rho)a vector of length containing the bias for scale and shape (in this order)
shape parameter
second-order parameter, non-positive
Dombry, C. and A. Ferreira (2017). Maximum likelihood estimators based on the block maxima method. https://arxiv.org/abs/1705.00465