We present a parametric study of electrostatic waves generated with angular frequencies 2 <b><i><font face="Symbol">w<sub>p,e</sub></font></i></b> and - <b><i><font face="Symbol">w<sub>p,e</sub></font></i></b> by an electron beam using a one-dimensional Vlasov code. We consider a background plasma consisting of three components: two electron populations (a background and a beam) and a proton population (with a mass ratio m<sub>p</sub> /m<sub>e</sub> = 400 and temperatures T<sub>p</sub> = T<sub>e</sub> = T ). We investigate the influence of different beam parameters on the nonlinear growth rate of waves with angular frequency 2<b><i><font face="Symbol">w<sub>p,e</sub></font></i></b> and compare the results of the numerical experiments to theoretical predictions. We also examine the presence and excitation of back propagating waves with angular frequency <b><i><font face="Symbol">w<sub>p,e</sub></font></i></b>. A discussion on the possible generating mechanisms of the different modes observed in these simulations is also presented.