Strongly bound doubly excited states of Be-2

.The triplet and singlet potential curves of Be-2 generated by single and double excitations from 2 sigma(u) into 3 sigma(g) and (or) 1 pi(u) are studied with a multireference configuration interaction (MRD-CI) method. Relative to X(1) Sigma(g)(+)(2 sigma(g)(2)2 sigma(u)(2)) with R(e) = 4.72 bohr and omega(e) = 258 cm(-1) (calculated here), these antibonding MO --> bonding MO excitations lead to average decreases in bond distance (in bohr) of 0.55 (2 sigma(u) --> 3 sigma(g)), 0.88 (2 sigma(u) --> 1 pi(u)), 0.93 (2 sigma(u)(2) --> 3 sigma(g)1 pi(u)), and 1.22 (2 sigma(u)(2) --> 1 pi(u)(2)). The increase in vibrational frequencies ranges from 240 to 600 cm(-1). The 3 sigma(g) MO is found to be less bonding than pi(u) confirming predictions made by Bader et al. The experimental states A(1) Pi(u) and B-1 Sigma(u)(+) correspond to doubly excited 1(1) pi(u)(2 sigma(u)(2) --> 3 sigma(g)1 pi(u)) and singly excited 1(1) Sigma(u)(+)(2 sigma(u) --> 3 sigma(g)), respectively. The 1(3) Sigma(g)(-) and 1(1) Delta(g) states, both 2 sigma(u)(2) --> 1 pi(u)(2), preserve their doubly excited structure up to dissociation. Within the Franck-Condon region, 1(3) Pi(u), changes from bound (2 sigma(u)(2) --> 3 sigma(g)1 pi(u)) to repulsive (mixed 2 sigma(u) --> 1 pi(g)/2 sigma(g) --> 1 pi(u)), thereby creating the unusual situation of a strongly bound potential (short R(e), high omega(e)) with an adiabatic dissociation energy near zero. The singlet counterpart 1(1) Pi(u), however, behaves regularly as its doubly excited character is maintained up to large R(Be-Be).