"We present a comparison of the optical nonlinear response calculations for bulk semiconductors, using length and transversal gauges. Ab initio calculations of the second-order optical susceptibilities for GaAs are presented as a case of study; calculations were performed using local density approximation (LDA) and to correct the underestimation of the band gap within density functional theory (DFT) scissors correction was implemented. Both formalisms were derived from the density matrix approach and the problem of gauge invariance is discussed."