DFT has been applied to evaluate electron paramagnetic resonance (EPR)g-tensor components of copper containing systems using an effective Spin-Orbit operator relying on the use of effective nuclear charges (Z(eff)). In particular, an optimal Z(eff) value was obtained for the copper atom by fitting the results obtained for a series of twenty copper bioinorganic complexes to the data obtained, at the same level of theory, using more refined approaches to evaluate Spin-Orbit Coupling terms. We showed that spin-orbit effects can be well approximated using the Z(eff) method giving reliable g-tensor values at low computational cost for large molecular compounds. (C) 2014 Elsevier B.V. All rights reserved.