The paper proposes an efficient method to compute the scattered electric field of a reconfigurable intelligent surface (RIS) for multiple configurations, accounting for mutual coupling between unit cells. Unlike existing methods that assume each unit cell scatters incident waves individually, this method considers the collective scattering and mutual coupling effects. The authors introduce the concept of effective unit cell complex radar cross-sections (CRCS) to accurately model the scattered fields, which are then used in ray tracing to simulate realistic, multipath radio environments with RISs. This approach addresses three critical considerations: mutual coupling between unit cells, multipath effects due to RIS as a diffuse scatterer, and the computational challenge of modeling RIS scattering for multiple configurations. The method is validated through comparisons with full-wave analysis and experimental results, demonstrating its accuracy and efficiency in predicting scattered field levels and link budgets for RIS-enabled communication channels.The paper proposes an efficient method to compute the scattered electric field of a reconfigurable intelligent surface (RIS) for multiple configurations, accounting for mutual coupling between unit cells. Unlike existing methods that assume each unit cell scatters incident waves individually, this method considers the collective scattering and mutual coupling effects. The authors introduce the concept of effective unit cell complex radar cross-sections (CRCS) to accurately model the scattered fields, which are then used in ray tracing to simulate realistic, multipath radio environments with RISs. This approach addresses three critical considerations: mutual coupling between unit cells, multipath effects due to RIS as a diffuse scatterer, and the computational challenge of modeling RIS scattering for multiple configurations. The method is validated through comparisons with full-wave analysis and experimental results, demonstrating its accuracy and efficiency in predicting scattered field levels and link budgets for RIS-enabled communication channels.