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Abstract

Due to the exponential increase in the numerical effort with the number of degrees of freedom, moving basis functions have a long history in quantum dynamics. In addition, spawning of new basis functions is routinely applied. Here, we advocate the opposite process: the programed removal of motional freedom of selected basis functions. This is a necessity for converged numerical results with respect to the size of a nonorthogonal basis because generically two or more states approach each other too closely early on, rendering unstable the matrix inversion, required to make the equations of motion explicit. Applications to the sub-Ohmic spin-boson model as well as to polaron dynamics in a Holstein molecular-crystal model demonstrate the power of the proposed methodology.