Electronic states of Pb5: geometries and energy separations

Geometries and energy separations of 24 low-lying electronic states of Pb5 with different structures are investigated employing the complete active space multiconfiguration self-consistent-field method (CASSCF) followed by multi-reference singles + doubles configuration interaction (MRSDCI) and the relativistic configuration interaction (RCI) computations that included up to 3.0 million configurations. The 1A′1 electronic state with trigonal bipyramidal (D3h) geometry is the ground state of Pb5, but it is mixed with triplet states about 8% due to spin-orbit coupling.