OCS in small para-hydrogen clusters: Energetics and structure with N=1–8 complexed hydrogen molecules

Abstract

We determine the structure and energetics of complexes of the linear OCS molecule with small numbers of para-hydrogen molecules, N =1-8, using zero temperature quantum Monte Carlo methods. Ground state calculations are carried out with importance-sampled rigid body diffusion Monte Carlo (IS-RBDMC) and excited state calculations with the projection operator imaginary time spectral evolution (POITSE) methodology. The ground states are found to be highly structured, with a gradual build up of two axial rings as N increases to 8. Analysis of the azimuthal density correlations around the OCS molecule shows that these rings are quite delocalized for small N values, but become strongly localized for N ≥ 5 . Excited state calculations are made for a range of total cluster angular momentum values and the rotational energy levels fitted to obtain effective rotational and distortion constants of the complexed OCS molecule as a function of cluster size N . Detailed analysis of these spectroscopic constants indicates that the complexes of OCS with para-hydrogen have an unusually rich variation in dynamical behavior, with sizes N =1-2 showing near rigid behavior, sizes N =3-4 showing extremely floppy behavior, and the larger sizes N =5-8 showing more rigid behavior again. The large values of the distortion constant D obtained for N =3-4 are rationalized in terms of the coupling between the OCS rotations and the "breathing" mode of the first, partially filled ring of para-hydrogen molecules.

Spectroscopy and dynamics in helium nanodroplets

“…The principle observation supporting this interpretation is the loss of Q branches in the spectrum of these complexes when upon cooling. Paesani et al [235,143] have performed theoretical calculations that provide support for this interpretation. One of us (KL) and others have argued that this observation is consistent with a set of pH 2 rings that can internally rotate (cyclic exchange) and the expected spin statistical weights of the internal rotation levels [6].…”

The ground state tunneling splitting and the zero point energy of malonaldehyde: A quantum Monte Carlo determination

“…This multiple exponential part of the decay, for very short imaginary time propagation, has been discarded 47 before performing the inverse Laplace transform of the truncated decay with the MaxEnt method ͑see Ref. 46 for details͒.…”

Infrared spectra of seeded hydrogen clusters: (para-H2)N–N2O and (ortho-H2)N–N2O, N=2–13

“…Nevertheless, the hunt for possible superfluidity in paraH 2 clusters has motivated a number of recent experiments involving (hydrogen) N -OCS clusters embedded within He nanodroplets and corresponding calculations. [20][21][22] Helium nanodroplets have also been used 19 to isolate small (HD) N -HCN clusters. Pure hydrogen clusters are of considerable interest, but more difficult to observed spectroscopically: infrared spectra of hydrogen dimers, (H 2 ) 2 , are well known, 24 calculations on hydrogen trimers, (H 2 ) 3 , have been reported, 25 and larger clusters have recently been observed by Raman spectroscopy.…”