Knowledge Bank

The electronic structure of the endohedral complex $Pa@C28$ was studied using ab initio quantum chemical methods including relativistic core potentials, gaussian double zeta basis sets, and spin-orbit configuration interaction calculations. As in $U@C28$ the self consistent field population analysis shows extensive mixing of Pa 6d and 5f orbitals with $C\pi$ orbitals, indicating strong binding. The energy of the $C28$ cage $\pi \ast$ orbitals (e symmetry) is lower than that of the Pa 5f orbitals. The ground state has an $e1$ configuration at both the SCF and CI level calculations. The lowest $f1$ state is 1.68 eV and 1.70 eV above the ground state from SCF and CI calculations respectively. Our previous $calculations1$ on $U@C28$ showed that the ground state of $U@C28$ is a $\pi \ast f$ state instead of an $f2$ state. The results for $Pa@C28$ support that finding.