Effects of strong electron correlations in Ti8C12 Met-Car

The results of multireference configuration interaction (MRCI) with single and double excitations and single reference coupled cluster (CCSD(T)) calculations on Ti8C12 metallocarbohedryne (Met-Car) are reported. The distortions of the Td structure to D2d and C3v structures due to the Jahn-Teller effect are studied. It is shown that the Ti8C12 wave function has significant multireference character. The choice of the active space for multireference self-consistent field (MCSCF) calculations is discussed. The failure of multireference perturbation theory with a small active space is attributed to multiple intruder states. A new, novel type of MCSCF calculation, ORMAS (occupation restricted multiple active spaces) with a large active space are carried out for several electronic states of Ti8C12. The Jahn-Teller distorted D2d1A1 (nearly Td) structure is predicted to be the Ti8C12 ground state. Predictions of the Ti8C12 ionization potential with different ab initio methods are presented.