In recent years we have developed a mathematical treatment of large amplitude collective motion in the adiabatic limit and formulated a set of methods, collectively known as the generalized valley approximation, that were applied to the approximate solution of a series of simplified models. In this paper we report the application of one of our algorithms to the study of the nucleus $^{28}\mathrm{Si}$, our first successful application to a realistic nuclear physics problem. We determine self-consistently a one-dimensional manifold of triaxial Slater determinants that connects the energy minimum of oblate deformation to the prolate minimum. Upon requantization of the implied collective Hamiltonian in the intrinsic frame, reasonable agreement with a shell-model calculation of the low-lying levels is achieved. Application of a theoretical criterion for assessing the quality of decoupling shows that a one-dimensional path is not sufficiently well decoupled in the model studied, thus suggesting one direction for future improvement. We compare our research with the only comparable previous work, that of Pelet and Letourneux.