The exact molecular reorientation model for nematic liquid crystals taking into account all diagonal Frank elastic constants and using two angles to describe director orientation is presented. Solutions and simplified equations are shown for the most common planar and chiral configurations. Gaussian beam propagation simulated using fully vectorial Beam Propagation Method in nonlinear case is also provided. Detailed comparison between exact solutions and single Frank constant approximation is made. However, no significant differences between these two models were found neither in beam propagation nor in polarization distribution, some difficulties may occur in choosing single Frank constant especially when it comes to quantitative results. Presented results correspond to a propagation of a beam of the Gaussian or topologically similar shapes.