Ground state expectation values are obtained by using a path integral ground state Monte Carlo method. The method allows calculations of ground state expectation values without the extrapolations often used with Green’s function and diffusion Monte Carlo methods. We compare our results with those of Green’s function Monte Carlo by calculating some ground state properties of the van der Waals complex He2Cl2 as well as the infinite systems liquid and solid He4. Advantages and disadvantages of the present method with respect to previous ones are discussed.

The Auxiliary Field Diffusion Monte Carlo method is applied to compute the spin susceptibility and the compressibility of neutron matter at zero temperature. Results are given for realistic interactions which include both a two-body potential of the Argonne type and the Urbana IX three-body potential. Simulations have been carried out for about 60 neutrons. We find an overall reduction of the spin susceptibilty by about a factor of 3 with respect to the Pauli susceptibility for a wide range of densities. Results for the compressibility of neutron matter are also presented and compared with other available estimates obtained for semirealistic nucleon-nucleon interactions and with more traditional many-body techniques, like Brueckner's or Correlated Basis Function theories.

The recently developed auxiliary field diffusion Monte Carlo method is applied to compute the equation of state and the compressibility of neutron matter. By combining diffusion Monte Carlo for the spatial degrees of freedom and auxiliary field Monte Carlo to separate the spin-isospin operators, quantum Monte Carlo can be used to simulate the ground state of many nucleon systems (A < ∼ 100). We use a path constraint to control the fermion sign problem. We have made simulations for realistic interactions, which include tensor and spin-orbit two-body potentials as well as three-nucleon forces. The Argonne v ′ 8 and v ′ 6 two nucleon potentials plus the Urbana or Illinois three-nucleon potentials have been used in our calculations. We compare with fermion hypernetted chain results. We report results of a Periodic Box-FHNC calculation, which is also used to estimate the finite size corrections to our quantum Monte Carlo simulations. Our AFDMC results for v6 models of pure neutron matter are in reasonably good agreement with equivalent Correlated Basis Function (CBF) calculations, providing energies per particle which are slightly lower than the CBF ones. However, the inclusion of the spin-orbit force leads to quite different results particularly at relatively high densities. The resulting equation of state from AFDMC calculations is harder than the one from previous Fermi hypernetted chain studies commonly used to determine the neutron star structure.

The structural and dynamical properties of OCS molecules solvated in Helium clusters are studied using reptation quantum Monte Carlo, for cluster sizes n = 3 − 20 He atoms. Computer simulations allow us to establish a relation between the rotational spectrum of the solvated molecule and the structure of the He solvent, and of both with the onset of superfluidity. Our results agree with a recent spectroscopic study of this system, and provide a more complex and detailed microscopic picture of this system than inferred from experiments. 61.46.+w, 67.40.Yv, 36.40.Mr, 02.70.Ss Solvation of atoms and molecules in He nanodroplets provides a way to study their properties in an ultra-cold matrix, and also offers a unique opportunity to probe the physics of quantum fluids in confined geometries. Research in this field has been recently reviewed in Ref.[1], with emphasis on experiment and hydrodynamic modeling, and in Ref.[2], with emphasis on computer simulations.Carbonyl sulfide (OCS) is one of the most widely studied dopants of He clusters (OCS@He n ) [3,4], both because of its strong optical activity in the infrared and microwave spectral regions, and also because the OCS@He 1 complex is spectroscopically well characterized [5,6], thus providing a solid benchmark for the atom-molecule interaction potential which is the key ingredient of any further theoretical investigation.In the quest of fingerprints of superfluidity in the spectra of small-and intermediate-size He clusters, Tang et al. have recently determined the vibrational and rotational spectra of OCS@He n at high resolution for n = 2 − 8 [3]. The main results of that investigation are: i) the rotational constant of the solvated molecule roughly equals the nanodroplet (largen) limit [7] at n = 5, but then undershoots this asymptotic value up to the maximum cluster size (n = 8) attained in that work; ii) the centrifugal distortion constant has a minimum at n = 5, thus indicating that the complex is more rigid at this size; iii) the fundamental vibrational frequency of OCS is not a monotonic function of the number of He atoms, but it displays a maximum at n = 5, again suggesting a stronger rigidity at this size. Findings ii) and iii) suggest-and our study of the rotational spectra confirms-that n = 5 is a magic size related to the structure of solvent atoms around the solvated molecule. Finding i) implies the existence of a (yet to be determined) minimum in the rotational constant as a function of the cluster size. The occurrence of this minimum was interpreted as due to quantum exchanges which would decrease the effective inertia of the first solvation shell and would thus be a signature of the onset of superfluidity in this finite system [3].Quantum simulations are complementary to the experiment for understanding the properties of matter at the atomic scale. In fact, while being limited by our incomplete knowledge of the inter-atomic interactions and by the size of the systems one can afford to examine, simulations provide a wealth of detailed information ...