Computational studies of microscopic superfluidity in 4he cluster

The physics that result in the decoupling of a molecule from a bosonic solvent at 0 K are studied. Fixed-node diffusion Monte Carlo (FNDMC) coupled with a Genetic Algorithm is used to perform simulations of the bosonic droplets doped with various molecules. The efficacy and accuracy of this approach is tested on a strongly coupled 2-dimensional quartic oscillator with excellent results. This al- gorithm is then applied to 4He-CO and 4He-HCN clusters respectively in an effort to determine the factors that result in the onset of microscopic superfluidity. The decoupling of the doped molecule from the bosonic solvent is found to be, primarily, a result of the combined effect of the repulsive interaction between the helium atoms and bose symmetry. The effects of rotor size versus molecular anisotropy in a NH3 molecule seeded into a 4He droplet is studied as well. Simulations are done using the accurate rotational constants (B0 = 9.945 cm--1, C0 = 6.229 cm--1 ) and using "fudged" versions of the rotational constants (Bfudged = 0.9945 cm--1, Cfudged = 0.6229 cm--1) for the |0011⟩ state. The simulations done with the fudged rotational constants experience a slightly smaller reduction than those done using the accurate rotational constants. This is attributed to the importance of molec- ular anisotropy versus the size of larger rotational constants in molecules whose rotational constants fall in an intermediate regime.