We present a combined experimental and computational study of the external heavy atom effect in van der Waals clusters of para-difluorobenzene (pDFB) with rare-gas atoms. Experimentally, clustering with rare-gas atoms is observed to shorten significantly the S-1 fluorescence lifetime compared with that of the pDFB monomer, an effect we interpret in terms of an enhancement of the S-1-T-1 intersystem crossing rate. In order to test the validity of this widely held assumption, we have calculated the S-1-T-1 spin-orbit coupling matrix elements in the X-pDFB complexes (X=Ne,Ar,Kr) using a multiconfigurational linear response approach. (C) 2004 American Institute of Physics.