Quantum structure and rotational dynamics of HCN in helium clusters

We present diffusion Monte Carlo calculations of ground states and rotationally excited states of HCN He-4(n), using our recently developed algorithm for importance sampled rigid body diffusion Monte Carlo [Viel , Comput. Phys. Commun. (in press, 2001)] within the mixed frame implementation. Excited states are studied with both fixed node approximations, and the Projection Operator Imaginary Time Spectral Evolution (POITSE) method that allows nodal constraints to be circumvented. Improvements in the POITSE algorithm allow excited states of clusters with up to 80 degrees of freedom to be determined here. The results presented here show that the rotational dynamics of the HCN molecule in He-4 clusters are very different from the behavior of heavier molecules such as SF6. Detailed analysis of ground state densities shows that the lighter HCN molecule induces negligible adiabatic following of the helium density as a result of its rotational motion. The excited state calculations show that for small numbers of He-4 atoms the nodal structure does not correspond to that of a freely rotating molecule. Nevertheless, the POITSE calculations indicate that there is some admixture of this nodal structure in the low-lying rotational excitations. It is found that a relatively large number of He-4 atoms are required to achieve saturation of the effective rotational constant at the experimental value, in contradistinction to the small numbers of atoms required to saturate the rotational constant for heavier molecules such as SF6 and OCS. (C) 2001 American Institute of Physics.