Coupled equations for the electric and magnetic fields describing pulse-envelope propagation in nonlinear, dispersive single-mode fibers in the frequency domain are presented. These equations contain frequency-dependent susceptibilities and conserve energy and momentum, thus enabling calculation of nonlinear, dispersive, and backscattering effects. Modeling of stimulated Raman scattering is improved by using a modified Sellmeier series. Two different pseudo-spectral algorithms are developed for simulation of signals propagating in one direction only and in both directions, respectively. Numerical results for the former are compared to those computed with the (generalized) nonlinear Schrödinger equation using the Lorentzian approach for describing stimulated Raman scattering. We show that significant differences between the models occur for bandwidths beyond 2.5 THz.