  The Mott insulator $\beta'$-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ belongs to a class
of charge transfer solids with highly-frustrated triangular lattice of $S=1/2$
molecular dimers and a quantum-spin-liquid ground state. Our experimental and
ab initio theoretical studies show the fingerprints of strong correlations and
disorder, important role of cation-dimer bonding in charge redistribution, no
sign of intra- and inter-dimer dipoles, and the decisive van der Waals
contribution to inter-dimer interactions and the ground state structure. The
latter consists of quasi-degenerate electronic states related to the different
configurations of cation moieties which permit two different equally probable
orientations. Upon reducing the temperature, the low-energy excitations slow
down, indicating glassy signatures as the cation motion freezes out.
