An ensemble model is proposed to describe the codeposition process of host and guest molecules. In this model, it is shown that the prepared matrix has a time related memory of the deposition process. A technique of time-going-backwards is developed to simulate the matrix structure around the guest molecule. In order to examine the reliability of this new method, a quantum fourth-order perturbation method is used for the first time to calculate the vibrational frequency shift of diatomic molecules in the matrix with respect to that in gas phase. The trapping site structures of some diatomic molecules (Br-2, BrO, CS, CN, ClO, NBr, and NCl) in solid argon and corresponding frequency shifts have been calculated with simple pairwise potentials. The simulation results are in good agreement with the experimental measurements.