Coupled-cluster approaches with an approximate account of triexcitations and the optimized-inner-projection technique. III. Lower bounds to the ground-state correlation energy of cyclic-polyene model systems

Second-order electronic correlation effects in a one-dimensional metal

“….) of D Mh symmetry, originated by Paldus and coworkers 58,59,60,61,62,63,64,65,66,67,68,69 (with some recent complements by the present authors 70,71,72,73 ). In the PPP model of cyclic polyenes (annulenes), the strength of the electronic correlation may be increased by (i) decreasing |β| for a fixed M , and/or (ii) by increasing M for a fixed β [parameter β(< 0) represents the so-called resonance integral of the PPP model].…”

“…57 Unfortunately, the coupled-cluster method is expected to fail when applied to the ground state of the 1D metal. This negative conclusion derives from the extensive CC studies of the PPP model of small cyclic polyenes ͑C M H M , M = 6 , 10, 14, 18, …͒ of D Mh symmetry, originated by Paldus and co-workers [58][59][60][61][62][63][64][65][66][67][68][69] ͑with some recent complements by the present authors [70][71][72][73] ͒. In the PPP model of cyclic polyenes ͑annulenes͒, the strength of the electronic correlation may be increased by ͑i͒ decreasing ͉␤͉ for a fixed M, and/or ͑ii͒ by increasing M for a fixed ␤ ͓parameter ␤͑Ͻ0͒ represents the so-called resonance integral of the PPP model͔.…”

Energy error bars in direct configuration interaction iteration sequence

“…were initiated by Löwdin 16 and were pursued for some time 8,[17][18][19]39 to become essentially abandoned to our date, with rare exceptions. 20 The bracketing function gives strict lower bound to an eigenvalue of the Hamiltonian, H, provided that the reference function, ϕ, is a normalized approximation to the state in question and ε is an appropriate upper bound.…”

Efficient Parallel Implementation of a Full Configuration Interaction Algorithm for Circular Polyenes On a Cray Y-Mp

“…Circular polyenes GNHN in the Pariser-Parr-Pople (PPP) Pople, 1953;Linderberg and Ohrn, 1968) approximation are well-known model systems of great importance in the theory of polymers, and a great deal of work has been done in order to obtain correlated wave functions for these systems (Paldus and Boyle, 1982;Paldus, Takahashi and Cho, 1984; Takahashi and Paldus, 1985;Piecuch et al, 1990). In particular, &dquo;exact&dquo; full configuration interaction (FCI) results are important benchmark calculations, to be compared with more practical approximated methods (see in particular the work of Paldus and associates cited above).…”