  We report the infrared spectrum of H$_2$C$_4$-(OCS)$_2$ trimer in the region
of the nu1 fundamental vibration of the OCS monomer. The van der Waals
complexes are generated in a supersonic slit-jet apparatus and probed using a
rapid-scan tunable diode laser. Both H$_2$C$_4$-(OCS)$_2$ and
D$_2$C$_4$-(OCS)$_2$ are studied. Analysis of their spectra establishes that
the trimer has C$_2$ point group symmetry. Theoretical calculations performed
to find stationary points on the potential energy surface confirm that the
observed structure is the most stable form. The experimental rotational
parameters are in very good agreement with those computed using double hybrid
functionals.
