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The quantum hard-sphere fluid with attractive Yukawa interactions is studied along several isochores, at low and medium densities, by varying the rest of the parameters that define its state points (the temperature, and the well-depth and inverse range of the attractive tail). Path integral Monte Carlo simulations are performed to fix the thermodynamics (energies and pressures) and the pair structures in r-space (instantaneous, linear response, and centroids). The k-space structure factors associated with these radial structures are determined via direct correlation functions defined by suitable Ornstein–Zernike equations. The results obtained allow one to assess quantitatively the reduction in the quantum features of the hard-sphere system due to the inclusion of attractive interactions. Besides, it is observed that under certain conditions the Yukawa interactions can lead to the formation of cavities in the system. For the sake of comparison, some complementary simulations of the bare quantum hard-sphere system have also been carried out, which have helped to resolve certain unclear features near a fluid–solid change of phase of this system. At the onset of increasing fluctuations in the Yukawa system one can observe, from the computation of k-space properties, the negligible role of the repulsive forces in this issue, and also the direct correlation function features associated with the attractions. In addition, the current quantum results give evidence of the decrease in both the critical temperature and the critical density of the Yukawa system, as compared with its classical counterpart.

The vibrational spectra of a synthetic purine nucleoside with known antiviral activity, 9-fl-D-arabinofuranosyladenine hydrochloride (ara-A.HC1) are reported. The Fourier transform infrared (FT-IR) and Fourier transform Raman (FT-Raman) spectra were recorded in the 4000-30 cm -1 spectral region. The harmonic frequencies and potential energy distributions (PED) of the vibrational modes of ara-A.HC1 were calculated by two different methods: a classical molecular mechanics method and a semiempirical molecular orbital (MO) method, PM3. The results of both computational methods, based on the Wilson GF method, are compared with observed spectra, and an assignment of the vibrational modes of ara-A.HC1 is proposed on the basis of the potential energy distributions (PED). It is found that the wavenumbers can be calculated with remarkable accuracy (=1% deviation in most cases), with the classical mechanics method, by transferring a sufficiently large set of available harmonic force constants, thus permitting a reliable assignment. The semiempirical MO method, PM3, is found to be useful for the assignment of experimental frequencies although it is less accurate (= 10% deviation). IR intensities calculated by this method did not coincide with the experimental values. Certain outof-plane vibrations in the base, not reported in previous studies, have been observed. The performance of both methods was related to the crystallographic and ab initio data available. Previous normal coordinate calculations for the adenine base and the nucleoside 5'-dGMP are compared with our results and discussed, in relation to the crystal structure of Ara-A.HC1.