Our solution to Eq. 5 is obtained by using the first order term of the Magnus expansion. The set of reference orbitals, obtained upon solving the HF equations in the present case, and the subsequent CI states built from the corresponding Slater determinants, remain unchanged during the TDCI propagation of the system's electronic wave function for a given molecular geometry. The explicit time-dependence is incorporated entirely through the time-dependent expansion coefficients in the many-body TDCI wave function.