  We present an ab-initio dynamical configuration interaction (DCI) study of
free- and Mg-porphyrin. DCI is a recently developed active space theory based
on the L\"owdin downfolding technique. In the active space, static correlation
is described exactly with full configuration interaction. In the high energy,
dynamically correlated subspace, we treat correlation at the quasiparticle
level in the $GW$ approximation of Green's function theory. The final theory
combines wave function and Green's function methods to give a balanced
description of static and dynamic correlation. The theory and algorithm give a
multireference treatment of ground and excited states for low computational
cost. The four orbital Gouterman model of porphyrin offers an ideal active
space in a large, correlated system to test the cost and accuracy of the
embedding for large systems. Our parameter free, fully ab-initio DCI
calculations in the minimal four-level active space agree well with both
experiment and more expensive benchmark theories for the $Q_x$ and $Q_y$
transitions of free- and Mg-porphyrin. Studying the convergence of the
excitation energies suggests that DCI correctly captures size extensive
correlation effects, making it a promising active space theory for large,
strongly-correlated systems.
