Exploring the parameter space of an endohedral atom in a cylindrical cavity

Abstract

Endohedral fullerenes, or endofullerenes, are chemical systems of fullerene cages encapsulating single atoms or small molecules. These species provide an interesting challenge of Potential Energy Surface (PES) determination as examples of non-covalently bonded, bound systems. While the majority of studies focus on C$_{60}$ as the encapsulating cage, introducing some anisotropy by using a different fullerene, e.g., C$_{70}$ can unveil a double well potential along the unique axis. By approximating the potential as a pairwise Lennard-Jones (LJ) summation over the fixed C cage atoms, the parameter space of the Hamiltonian includes three tunable variables: $(M,\varepsilon,\sigma)$ representing the mass of the trapped species, the LJ energy, and length scales respectively. Fixing the mass and allowing the others to vary can imitate the potentials of endohedral species trapped in more elongated fullerenes. We choose to explore the LJ parameter space of an endohedral atom in C$_{70}$ with $\varepsilon\in$ [20cm$^{-1}$, 150cm$^{-1}$], and $\sigma\in$ [2.85\r{A} , 3.05\r{A}]. As the barrier height and positions of these wells vary between [1cm$^{-1}$, 264cm$^{-1}$] and [0.35\r{A}, 0.85\r{A}] respectively, using a 3D direct product basis of 1D harmonic oscillator (HO) wavefunctions centred at the origin where there is a local maximum is unphysical. Instead we propose the use of a non-orthogonal basis set, using 1D HO wavefunctions centred in each minimum and compare this to other choices. The ground state energy of the X@C$_{70}$ is tracked across the LJ parameter space, along with its corresponding nuclear translational wavefunctions. A classification of the wavefunction characteristics, namely the prolateness and ``peanut-likeness'' based on its statistical moments is also proposed.