Аннотация:We perform ab initio no core shell model calculations for A=18 and 19 nuclei in a $4\hbar\Omega$, or $N_{\rm max}=4$, model space using the effective JISP16 and chiral N3LO nucleon-nucleon potentials and transform the many-body effective Hamiltonians into the $0\hbar\Omega$ model space to construct the A-body effective Hamiltonians in the sd-shell. We separate the A-body effective Hamiltonians with A=18 and A=19 into inert core, one- and two-body components. Then, we use these core, one- and two-body components to perform standard shell model calculations for the A=18 and A=19 systems with valence nucleons restricted to the sd-shell. Finally, we compare the standard shell model results in the $0\hbar\Omega$ model space with the exact no core shell model results in the $4\hbar\Omega$ model space for the A=18 and A=19 systems and find good agreement.