Conclusions A reliable and tractable technique for constructing the ground-state wave function by the superposition of nonorthogonal SDs is described. Linear independent multiple correction vectors are employed in order to update one-electron wave functions, and a conventional steepest descent method is also performed as a comparison. The dependence of convergence performance on the number of adopted correction vectors is also illustrated. The electron–electron correlation energy converges rapidly and smoothly to the ground state through the multi-direction search, and an essentially exact ground-state energy is obtained with drastically fewer SDs (less than 100 SDs in
the present see more study) compared with the number required in the full CI method. For the few-electron molecular systems considered in the present study, essentially exact electron–electron correlation energies can be calculated even at
long bond lengths for which the standard single-reference CCSD and CCSD(T) show poor results, and the practicality and applicability of the proposed calculation procedure have been clearly demonstrated. In future studies, calculations employing periodic boundary conditions and effective core potentials (ECPs) this website  will be performed. A new procedure to reduce the iteration cost should be found in order to increase the applicability of the proposed algorithm for the calculation of essentially exact ground-state energies of many-electron systems. Acknowledgments The present study was partially supported by a Grant-in-Aid for the Global COE Program ‘Center of Excellence for Atomically Controlled Fabrication Technology’ (grant no. H08), for a Grant-in-Aid for Scientific Research on Innovative Areas ‘Materials Design through Computics: Complex Correlation and Non-Equilibrium Dynamics’ (grant no. 22104008), a Grant-in-Aid for Scientific Research in Priority Areas ‘Carbon Nanotube Nano-Electronics’
(grant no. 19054009) and a Grant-in-Aid for Scientific Research (B) ‘Design of Nanostructure Electrode by Electron Transport Simulation for Electrochemical FDA-approved Drug Library Processing’ (grant no. 21360063) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. References 1. Palmer IJ, Brown WB, Hillier IH: Simulation of the charge transfer absorption of the H 2 O/O 2 van der Waals complex using high level ab initio calculations. J Chem Phys 1996, 104:3198.CrossRef 2. Kowalski K, Piecuch P: The method of moments of coupled-cluster equations and the renormalized CCSD[T], CCSD(T), CCSD(TQ), and CCSDT(Q) approaches. J Chem Phys 2000, 113:18.CrossRef 3. Gwaltney SR, Sherrill CD, Head-Gordon M: Second-order perturbation corrections to singles and doubles coupled-cluster methods: General theory and application to the valence optimized doubles model. J Chem Phys 2000, 113:3548.CrossRef 4.