A washboard with moment of inertia (WBMI) model for gas atom scattering from a flexible surface is proposed and applied. This model is a direct extension of the washboard model [J. Chem. Phys. 92, 680 (1990)] proposed for gas atom scattering from relatively rigid, corrugated surfaces. In addition, a moment of inertia is incorporated in the original washboard model to describe the flexibility of softer, more highly corrugated surfaces such as polymer or liquid surfaces. The moment of inertia of the effective surface object introduces a dependence of the efficiency of energy transfer on the position and direction of impact, a feature that has been shown to be critical by molecular dynamics simulations. The WBMI model is solved numerically by Monte Carlo integration, which makes the implementation of multiple impacts between a colliding atom and the surface very efficient. The model is applied to Ne and Ar atoms scattering from an alkylthiolate self-assembled monolayer surface and reproduces the major results obtained by classical trajectory simulation of the same system, i.e., a bimodal translation energy distribution P(E-f) with the low-energy component well-fit with a Boltzmann distribution, but with a temperature that may (Ar) or may not (Ne) be the same as the surface temperature. This indicates that the WBMI model, with well-motivated physical assumptions and simplified interaction, reveals many of the major aspects of the gas-surface collision dynamics, though it does not take into account the real-time dynamics explicitly. (C) 2004 American Institute of Physics.