Theoretical study of Be linear chains: optimized geometries and harmonic frequencies

The electronic structure of linear Beryllium chains has been theoretically studied at an ab initio level. By using a CAS-SCF approach, geometries have been optimized and harmonic frequencies computed. The optimized geometries present almost equal bond lengths, while all the harmonic frequencies are real. This fact indicates the presence of a local minimum, at this level of theory, having a linear geometry. The energy splitting between the singlet ground state, 1Sigma_g , and the quasi-degenerated excited triplet, 3 Sigma_u, has been computed at CAS-SCF and MR-CI level. It is found that the splitting goes exponentially to zero as the number of atoms in the chain is increased.