28 April 2000 Ž . Chemical Physics Letters 321 2000 216–224 www.elsevier.nlrlocatercplett High-order coupled-cluster calculations through connected octuple excitations So Hirata, Rodney J. Bartlett ) Quantum Theory Project, UniÕersity of Florida, GainesÕille, FL 32611, USA Received 3 January 2000; in final form 24 February 2000 Abstract Ž . By exploiting a determinantal full configuration interaction FCI algorithm, we compute the correlation energies of Ž . Ž . molecules at any arbitrary order of coupled-cluster CC theory as well as high orders of configuration interaction CI and Ž . many-body perturbation theory MBPT . This general-order CC program requires memory storage for three arrays of length Ž . N the number of determinants and a modest amount of disk storage. We perform the CC calculations including all det connected n-fold excitations up to n s8 for H O, FH, and F y . q 2000 Elsevier Science B.V. All rights reserved. 2 1. Introduction Ž . w x Configuration interaction CI theory 1,2 , Ž . w x many-body perturbation theory MBPT 3–6 , and Ž . w x coupled-cluster CC theory 6–10 offer systematic routes to improving the wavefunctions and energies of atoms and molecules in their ground electronic w x state. The CI method 1,2 , which is conceptually the simplest among them, allows the reference configu- ration to mix with single, double, etc. excitation configurations and determines the mixing coeffi- Ž . cients the CI coefficients variationally. A serious deficiency of this method is that the wavefunctions and energies obtained in this way are not size-exten- w x sive 6,10 unless all possible excitation configura- tions are included. When all configurations are in- cluded, the method becomes full configuration inter- Ž . action FCI , and several results have been reported ) Corresponding author. Fax: q 1-352-392-8722, e-mail: bartlett@qtp.ufl.edu w x 11–17 . Despite its very limited applicability, FCI is recognized as an indispensable computational method for benchmarks, as it provides the best possible wavefunction and energy of a system within a given one-particle basis set. w x The MBPT method 3–6 treats electron correla- tion as a perturbation to the independent particle reference. The MBPT method truncated at any per- turbation order is size-extensive and the MBPT cal- w x culations through fourth order 18,19 employing the Møller–Plesset partitioning scheme are routine. Ž. w x MBPT has also been implemented at MBPT 5 20 Ž. w x and MBPT 6 in a general MBPT program 21,22 . The FCI method also makes it possible to compute the MBPT energies order by order and to study the convergence of the perturbation series, provided a FCI calculation is possible as demonstrated by Laidig w x w x et al. 23 and by others 24,25 . The convergence of the perturbation series is slow and oscillatory in w x many cases, and Olsen and co-workers 25,26 find that certain situations lead to divergent behavior in 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 00 00387-0