A simulation method is proposed which approximates the atmospheric beam path as an extremely large aperture hollow waveguide containing a numerically sufficient subset of linearly polarized lossless propagation modes. The proposed method is shown to agree numerically with the standard angular spectrum of plane waves method in a single transverse dimension simulation and is readily expandable into two transverse dimensions but currently limited by available hardware. The method is expanded to involve coupling matrices describing the transition through each phase screen and the intermediate free-space portions. The coupling matrices are combined into a single multimode coupling matrix describing the propagation for one instance of atmosphere. The proposed matrix method has potential to evaluate multiple input beams simultaneously or condense high turbulence simulations requiring many phase screens. The input beam can be constructed by multiplying the decomposition of the desired output profile with the pseudo-inverse of the coupling matrix; however, the matrix cannot be realized in experiment. Therefore, the opportunity for beam shaping to compensate optical turbulence is evaluated by principal component analysis of the compound coupling matrix. It is shown that an average of the lowest order eigenmode across multiple simulations produces a super-Gaussian-like beam with improved power delivery and stability. The implication for optical countermeasure beam control is the potential to create any desired beam shape at the target plane.