ELSEVIER 21 November 1997 Chemical Physics Letters 279 (1997) 403-410 CHEMICAL PHYSICS LETTERS Theoretical study of the electronic states of Zr 5 D. Majumdar, K. Balasubramanian Department ~f"Chemisto' and Biochemisto', A ri:ona State Unicersity, Tempe, AZ 85287-1604, USA Received 20 August 1997 Abstract Geometries and energy separations of low-lying electronic states of Zr5 with different structures have been investigated by complete active space multi-configuration self-consistent field method followed by large-scale multi-reference singles + doubles configuration interaction computations that included up to 3 million configurations. Three nearly degenerate electronic states, namely 3A 2 (C2,.) with a distorted tetragonal pyramid structure and J,~ and 57~ (C~) with distorted edge capped tetrahedral geometries were found as the candidates for the ground state of Zr5. The atomization and dissociation energies have been calculated and compared with smaller clusters. © 1997 Elsevier Science B.V. 1. Introduction Electronic structural properties of small transition-metal clusters have been the subject of prime importance for several years. Large number of electronic states with varied spin multiplicities result- ing from the open d shells have made the electronic and structural properties of the transition-metal clus- ters challenging to both theorists and experimental- ists [1-11]. The interplay of electron correlation effects, spin-exchange stabilization energies and chemical bonding is so intricate that it becomes extremely difficult to predict the ground state of these species. Clusters of zirconium are interesting from a theo- retical standpoint as a result of participation of both 5p and 4d orbitals of Zr atom in addition to the 5s orbital in bonding [5]. Extensive theoretical studies on Zr, [5], Zr 3 [7] and Zra [8] clusters have revealed that electron correlation effects are so significant that the relative ordering of different electronic states varies as a function of the level of theory. In recent years the 4d-transition-metal clusters have further drawn attention because of their promising applica- tion in developing new magnetic materials [12-15]. In case of Zr clusters, it has been found that the they have magnetic properties up to a certain cluster size limit [14,15]. In the present investigation we have carried out systematic complete active space multi-configuration self-consistent field (CASMCSCF) followed by multi-reference singles + doubles configuration in- teraction (MRSDCI) computations of the low-lying electronic states of Zr 5. Full geometry optimization has been carried out at the CASMCSCF level using quasi Newton-Raphson technique [16]. The low-lying electronic states were further optimized at the MRSDCI level of calculations. Our investiga- tions at the highest level of theory revealed that the low-lying electronic states of this cluster are mostly singlets and triplets with distorted geometries. 2. Methods of computation The geometrical arrangements considered for Zr 5 are shown in Fig. I. The various geometries included 0009-2614/97/$17.00 ~') 1997 Elsevier Science B.V. All rights reserved. Pll S0009-2614(97)01057-9