Theoretical Simulations Of The Structures And Superfluidity Of Van Der Waals Clusters Containing N₂O

Spectroscopic studies of probe molecules embedded in low-temperature rare gas clusters have revealed fascinating structural and dynamical behaviors of the micro-solvation environment. The experimental observations and theoretical simulations of the doped HeN clusters could provide valuable information for the transition from the rotation of a weakly bond van der Waals complex to the quantum solvation of an impurity solved in a superfluid helium nanodroplet. Due to the sensitivity of the spectroscopy to the nature of the interaction between the solvent and impurity, an accurate dimer potential energy surface (PES) is particularly desirable for understanding the quantum insight of the observed structural and dynamic properties of doped clusters. In addition, an efficient quantum simulation method is needed to describe quantum fluctuation in these relatively large systems at a finite temperature. In this work, we explicitly include the dependence of two intramolecular vibrational coordinates which are related to the infrared spectra of the N2O chromophore in such clusters to construct new potential energy surfaces for He-N20and H2-N2O complexes. The path integral Monte Carlo (PIMC) method which considers the rotational degree of freedom of the chromophore impurity is used for the simulation of the HeN-N2O and (para-H2)N-N2O clusters. The experimental observations of spectroscopy and superfluidity are reproduced with quantum mechanical insight in our simulations. While the studies of the superfluidity of HeN-N2O clusters have been widely reported, the different effective rotational constants of HeN-N2O and HeN-CO2clusters have not been quantum mechanically explained. The theoretical understanding of the observed evolution of v3band origin shift is also undetermined. Previously reported results have shown that a three-dimensional He-N20dimer potential energy surface (PES) failed to provide precise prediction of the v3band origin shift for He-N20dimer. It has been proved in our work that the vibrational coordinates Q1of N2O should be explicitly involed due to the strong coupling between the symmetric and asymmetric stretches of N2O. To this end, the potential energy surface for He-N20is constructed four-dimensionally at CCSD(T) level with aug-cc-pVQZ basis set together with bond functions. The ab initio potential points are fitted to a four-dimensional Morse/Long Range (MLR) analytical form to obtain the global potential energy surface, and the ab initio noise in the long range region of the potential is smoothed over in the fitting by theoretically fixed long range parameters. By vibrationally averaging the four-dimensional potential, two-dimensional intermolecular potentials for both the ground and the excited v3states of N2O are then constructed. Based on the two-dimensional potentials, a finite temperature path integral Monte Carlo study of the structural, dynamical properties and superfluidity of HeN-N2O clusters is presented. The first-order perturbation theory estimate is used in our presented work to evaluate the band original shift efficiently. The distribution of solvent atoms, the shift of the N2O antisymmetric stretching (v3) band origin, as well as the effective rotational constants which provide insight into superfluidity are investigated as a function of the cluster size. The evolution tendency from dimer to large clusters is excellently reproduced compared with experimental observations. Furthermore, a careful comparison with the rotational dynamic properties of HeN-CO2clusters suggests that the difference between the effective rotational constants of the two impurity molecules is due to the anisotropy of the solute-solvent interaction potential. The para-H2molecules are spinless indistinguishable bosons, so they are expected to behave similarly as4He at low temperature. Up to date, the quantum insight of the observed evolution of the vibrational band origin shift of (para-H2)N-N2O clusters is still left undetermined, and the onset of superfluidity in such clusters is still unclear. We construct a six-dimensional ab initio potential energy surfaces for the H2-N2O complex that explicitly incorporated the vibrational coordinates Q1and Q3of N2O. Analytic four-dimensional PESs are then obtained by least-squares fitting the vibrationally averaged interaction energies for v3(N2O)=0, and1to the four-dimensional MLR potential function form. The rotation of H2is handled using a hindered rotor model. Using the path integral Monte Carlo (PIMC) method, the structural and dynamical propertied of (para-H2)N-N2O clusters are studied quantum mechanically based on the obtained (para-H2)-N2O PESs. In good agreement with experimental observations, the N2O vibrational band origin shifts shows a turnaround near N=5. The observed reductions in the shift rate from N=4to5which is unique compared to He are also reproduced. In addition, for clusters sized N-1to13, no feature of superfluidity arise in the study of rotational dynamics of (para-H2)M-N2O clusters, which is the same as the experimental observations.Besides, the structures and spectroscopic properties of Ar-CO2complex also has been the subject of much attention and research in the van der Waals cluster community. Although these cold clusters would not exhibit superfluidity, they provided a useful bridge between van der Waals complexes and bulk solutions. Using path integral Monte Carlo method, the ArN-CO2clusters are simulated quantum mechanically in our presented work to properly incorporate the quantum effects of these clusters at low temperatures. The simulations of ArN-CO2clusters are based on a newly developed analytical Ar-CO2interaction potential obtained by fitting vibrational averaged ab initio results to the anisotropic two-dimensional MLR function. The calculated distributions of solvation atoms around the dopant molecule in ArN-CO2clusters with different sizes agree with the previous studies of the configurations of the clusters. The CO2vibrational frequency shift is quantitatively predicted in PIMC simulation using a first-order perturbation theory and agrees with experimental observations. After the completion of the first-sol vati on shell at N=17, the simulations for larger ArN-CO2clusters showed several different structures of the argon shell around the doped CO2molecule. The previously observed two distinct peaks (2338.8and2344.5cm-1) in the v3band of CO2in argon matrix may be due to the different arrangements of solvation atoms around the dopant molecule.