Complete basis set extrapolations and model chemistries.

摘要

A sequence of n-tuple-&zgr; augmented polarized ( nZaP, n=2--6) basis sets designed for extrapolations of both self-consistent field (SCF) and correlation energies to the complete basis set (CBS) limit are constructed. The basis sets are formulated to give consistent errors throughout the Periodic Table. The SCF energy systematically converges to the CBS limit: ESCF(nZaP) ≈ ESCF(CBS) + Ae-an . A single parameter, a = 6.30, describes the 2ZaP--6ZaP H--Xe errors within 10%. Linear extrapolations of the (n, n + 1)ZaP calculations (all with a = 6.30) reduce the errors by an order of magnitude.;Re-examination of the basis set convergence of the separate SCF, MP2-alphabeta, MP2-alphaalpha, CCSD--MP2, and (T) components of the valence CCSD(T) energy for Neon with the nZaP basis sets gives us an improved benchmark for the CCSD(T) energy: --128.869236 +/- 0.00002 hartrees. We then extrapolate to the MP2/CBS limit with nZaP for 72 atoms, atomic ions, homonuclear diatomic molecules, and hydrides of H--Ar. The second-order correlation energies of the closed-shell atoms agree with accurate (+/-0 01 mEh) numerical values to within +/-0.1%. These MP2/CBS limits can now be used as benchmarks to calibrate more approximate calculations using smaller basis sets.;A restricted-open-shell model chemistry based on the complete basis set-quadratic Becke3 (ROCBS-QB3) model is evaluated. ROCBS-QB3 eliminates the spin correction in standard CBS-QB3 with no loss in accuracy, and is recommended over standard CBS-QB3 for open-shell systems. Unrestricted coupled cluster [UCCSD(T)], spin contamination corrected UCCSD(T), and unrestricted Brueckner doubles [UBD(T)] variations of Weizmann-1 theory (W1) are compared with restricted-open-shell W1 theory [W1(RO)]. The four methods are indistinguishable in performance, except for the pathological cases C2, O3 and the dissociation curve of F2. These demonstrate that W1BD has two significant advantages over the other variants: (i) the error in ⟨S 2⟩BD correlates with the magnitude of the energy error, unlike in W1(RO), and (ii) W1BD is much less spin-contaminated than W1U. Learning from the strengths and weaknesses of previous models, a new generation of CBS methods is currently under development. Promising results are presented for "CBS-Wesleyan Single Polarization" (CBS-Wes1P), which uses one level of polarization for the most expensive triples step. The model achieves W1 accuracy for the frozen-core CCSD(T) energy at a cost that is two orders-of-magnitude less.