  In this second part of our series on the recently proposed many-body expanded
full configuration interaction (MBE-FCI) method, we introduce the concept of
multideterminantal expansion references. Through theoretical arguments and
numerical validations, the use of this class of starting points is shown to
result in a focussed compression of the MBE decomposition of the FCI energy,
thus allowing chemical problems dominated by strong correlation to be addressed
by the method. The general applicability and performance enhancements of
MBE-FCI are verified for standard stress tests such as the bond dissociations
in H$_2$O, N$_2$, C$_2$, and a linear H$_{10}$ chain. Furthermore, the benefits
of employing a multideterminantal expansion reference in accelerating
calculations of high accuracy are discussed, with an emphasis on calculations
in extended basis sets. As an illustration of this latter quality of the
MBE-FCI method, results for H$_2$O and C$_2$ in basis sets ranging from double-
to pentuple-$\zeta$ quality are presented, demonstrating near-ideal parallel
scaling on up to almost $25000$ processing units.
