Molecular dynamics simulations of liquid water using a rigid molecular model provide statistical characteristics of the forces acting on the internal degrees of freedom. This determines the perturbation of the internal oscillators, from which fundamental spectra can be derived. It turns out that the spectra are determined by the distribution of forces, causing a shift and an inhomogeneous broadening due to anharmonicity of the oscillators. This broadening and shift are related to the number and strength of the hydrogen bonds in which the molecule participates as a hydrogen bonding donor. Further causes of spectral perturbation (homogeneous broadening, intermolecular coupling, modulation by rotational motion) are negligeable. Temperature dependence of simulated spectra produce an isosbestic point, as observed experimentally, indicating the validity of a two state description.