An ab initio model of scanning tunneling microscopy (STM) is presented in the framework of the perturbation theory of Bardeen. The tip and sample are represented by the cluster model, and the first-order wave functions are calculated by ab initio Hartree-Fock (HF), configuration interaction (CI), and symmetry adapted cluster (SAC)/SAC-CI methods. This model was applied to simple tip-sample systems such as-Pd-2-Ag-2, Pd-2-C6H6, Pd-2-(C6H6)(2), and Li-2-Li-2. The calculated STM images were related to the HOMO, LUMO, etc, orbital symmetries of the tip-sample systems. The contribution of the Rydberg orbital was examined at different tip-sample distances. Electron correlations were found to modify the behavior of the tunneling transition probability of the Li-2-Li-2 system vs that calculated by the HF method.