“…According to recent work on positron annihilation rates (Gribakin, 2000(Gribakin, , 2001, a high density of VFRs, growing exponentially with the number of atoms in the molecule, is required in order to explain the measured annihilation rates. This was confirmed by recent experiments of Barnes et al (2003) which indicate a strong correlation between the spectrum of vibrational modes and the annihilation spectrum for propane and heptane (see Fig.…”