  We are concerned with improving the diagnostic potential of the K lines and
edges of elements with low cosmic abundances that are observed in the X-ray
spectra of supernova remnants, galaxy clusters and accreting black holes and
neutron stars. Since accurate photoabsorption and photoionization cross
sections are needed in their spectral models, they have been computed for
isoelectronic sequences with electron number $12\leq N\leq 18$ using a
multi-channel method. Target representations are obtained with the atomic
structure code AUTOSTRUCTURE, and ground-state cross sections are computed with
the Breit--Pauli $R$-matrix method (BPRM) in intermediate coupling, including
damping (radiative and Auger) effects. The contributions from channels
associated with the 2s-hole $[2{\rm s}]\mu$ target configurations and those
containing 3d orbitals are studied in the Mg and Ar isoelectronic sequences.
Cross sections for the latter ions are also calculated in the
isolated-resonance approximation as implemented in AUTOSTRUCTURE and compared
with BPRM to test their accuracy. It is confirmed that the collisional channels
associated with the $[2{\rm s}]\mu$ target configurations must be taken into
account owing to significant increases in the monotonic background cross
section between the L and K edges. Target configurations with 3d orbitals give
rise to fairly conspicuous unresolved transition arrays in the L-edge region,
but to a much lesser extent in the K-edge which is our main concern; therefore,
they have been neglected throughout owing to their computationally intractable
channel inventory, thus allowing the computation of cross sections for all the
ions with $12\leq N\leq 18$ in intermediate coupling with BPRM. We find that
the isolated-resonance approximations performs satisfactorily.
