Infinite basis limits in electronic structure theory

We have developed a database of 29 molecules for which we have estimated th e complete-one-electron-basis-set limit of the zero-point-exclusive atomiza tion energy for five levels of electronic structure theory: Hartree-Fock (H F) theory, Moller-Plesset second- and fourth-order perturbation theory, cou pled cluster theory based on single and double excitations (CCSD), and CCSD plus a quasiperturbative treatment of triple excitations [CCSD(T)], all at a single set of standard geometries. Convergence checks indicate that the estimates are within a few tenths of a kcal/mol of the n=infinity limit of the cc-pVnZ basis set sequence. This data is then used to obtain optimized power-law exponents for extrapolating to the basis-set-limit from correlati on-consistent polarized valence double and triple zeta (cc-pVDZ and cc-pVTZ ) basis sets. This allows one to get thermochemical accuracy comparable to polarized quadruple or quintuple zeta (cc-pVQZ or cc-pV5Z) basis sets with a cost very comparable to polarized triple zeta, which is one order of magn itude less expensive than polarized quadruple zeta and two orders of magnit ude less expensive than polarized quintuple zeta. (C) 1999 American Institu te of Physics. [S0021-9606(99)30829-1].