- Author
-
Petersson, G. A.
- Title
- Pair Natural Orbital Extrapolations to the Complete Basis Set Limit.
- Coporate
- Wesleyan Univ., Middletown, CT
- Report
-
Video,
- Keywords
-
- Abstract
- The major source of error in most ab initio calculations of molecular energies is the truncation of the one-electron basis set. Analysis of the helium-like ion with infinite nuclear charge provides a model for this basis set truncation error. Extrapolation to the complete basis set (CBS) limit using this model N⁻¹ asymptotic convergence of N-configuration pair natural orbital (PNO) expansions can be combined with the use of relatively small basis sets for the higher-order (i.e. MP3, MP4 and QCI) correlation energy to develop cost effective computational methods, denoted CBS-4, CBS-Q and CBS-QCI/APNO. The RMS errors of these methods for the 125 chemical energy differences of the G2 test set are 2.5, 1.3, and 0.7 kcal/mol, respectively. An extension of these models to the characterization of transition states (TS) for chemical reactions gives RMS errors in the classical barrier heights for ten atom exchange reactions of 1.3, 0.6 and 0.3 kcal/mol, respectively. The CBS-QCI/APNO AX-VTST absolute rates for five hydrogen abstraction reactions with barrier heights from 1.3 kcal/mol (H₂ + F) to 20 kcal/mol (H₂O + H) have been calculated over temperatures from 250 K to 2500 K. These calculated rate constants, ranging from 10⁻⁸ to 10-10 cm³ molecule⁻¹ sec⁻¹, are all within the uncertainty of the experiments. New CBS/PNO results for the relativistic and nonrelativistic limits for third-row atoms and ions provide IPs and EAs within 0.02 eV of experiment. This residual error is much smaller than the relativistic contraction and spin-orbin corrections (up to 0.6 eV each).