A mathematical model which is logically very simple and convenient for programming is proposed for describing and studying the optical spectra of polyatomic molecules from the IR to the UV, including the Rydberg region, with computer simulations. For a specified molecular geometry, the only variable parameters are the elements of the force constant matrices for the ground and electronically excited states and the well depths. The functions for the electrons are specified by sets of principal quantum numbers for the radial components in a hydrogen-like atom. The model can be used to solve both direct and inverse problems.