We have devised a local ab initio density matrix renormalization group algorithm to describe multireference correlations in large systems. For long molecules that are extended in one of their spatial dimensions, we can obtain an exact characterization of correlation, in the given basis, with a cost that scales only quadratically with the size of the system. The reduced scaling is achieved solely through integral screening and without the construction of correlation domains. We demonstrate the scaling, convergence, and robustness of the algorithm in polyenes and hydrogen chains. We converge to exact correlation energies ͑in the sense of full configuration interaction, with 1-10 E h precision͒ in all cases and correlate up to 100 electrons in 100 active orbitals. We further use our algorithm to obtain exact energies for the metal-insulator transition in hydrogen chains and compare and contrast our results with those from conventional quantum chemical methods.

To assess the relative energies and free energies of five canonical and three zwitterionic low-lying structures of the arginine molecule, modern basis set extrapolation techniques and high-level ab initio treatments of electron correlation have been used on state-of-the-art parallel computers. The electronic energy and Gibbs free energy orderings of these eight species turn out to be consistent with previous findings [Rak, J.; Skurski, P.; Simons, J.; Gutowski M. J. Am. Chem. Soc. 2001, 123, 11695] obtained using smaller basis sets and lower level treatments of electron correlation. Nevertheless, the results presented here represent what the current state of the art can achieve for a molecule of this size and complexity and they offer the best available estimates of the relative stabilities of the eight structures.

We compute the potential-energy curve of the hydrogen fluoride molecule (HF) using a novel variant of the explicitly correlated multireference averaged coupled-pair functional method with a carefully selected basis set and reference space. After correcting for scalar relativistic effects and spin-orbit coupling, the potential is used to compute the dissociation energy, the equilibrium bond distance, the harmonic frequency, the anharmonicity, and the vibrational levels up to the dissociation limit. The errors in the equilibrium geometry constants compare favorably with the most elaborate (single reference) calculations of the literature. Starting at the region of RA/angstroms approximately 2,...,3, where the covalent HF bond begins to break and where single-reference methods become impractical, our potential begins to slightly underestimate the atomic interaction, which is reflected in an estimated error in the well depth of -0.2 kcal/mol.

ABSTRACT:A new electronic configuration reference space (subsequently used in multi-reference averaged coupled pair functional (MR-ACPF) or multi-reference configuration interaction singles and doubles [MR-CI(SD)] level treatments of electron correlation) is determined using the aug-cc-pVQZ basis set as a step toward constructing a new potential energy surface (PES) for the F ϩ H 2 3 FH ϩ H reaction. We use our new reference space to calculate several chemically important properties (e.g., barriers, exothermicity, van der Waals wells) of the F ϩ H 2 PES. We obtain nonrelativistic barrier heights of 1.32 kcal/mol Ϫ1 (bent) and 1.67 kcal/mol Ϫ1 (collinear) that are ϳ0.2-0.3 kcal/ mol Ϫ1 lower than those obtained from the current best PES. Our nonrelativistic value for the exothermicity is 32.45 kcal/mol Ϫ1 , which is 0.7-1.1 kcal/mol Ϫ1 higher than the values obtained from some other PESs and 0.45 kcal/mol Ϫ1 higher than the experimental value. The van der Waals wells we find are slightly deeper (0.05 kcal/mol Ϫ1 ) than the wells on the other PES. The Ϸ1-kcal/mol Ϫ1 magnitude of the differences among barrier heights, exothermicities, and well depths, obtained in our work and using the most reliable current PESs suggest that to obtain a three-dimensional (3D) PES for the F ϩ H 2 3 FH ϩ H accurate to 0.2 kcal/mol Ϫ1 , we will have to use even higher-level methods (e.g., explicitly correlated wave functions) and include relativistic corrections. We intend to do so in the next phase of this effort that is currently under way.

Explicitly correlated averaged coupled-pair functional methods have been used to compute the ground-state Born-Oppenheimer potential energy surface for the F + HH' --> FH + H' reaction at the F + HH' and FH + H' asymptotes, the F...HH', and FH...H van der Waals wells, the reaction transition state, and at points along the intrinsic reaction coordinate connecting all of these stationary points. To these energies, corrections for spin-orbit coupling and scalar relativistic effects were added to produce total electronic energies whose accuracy is demonstrated to be very high (e.g., 0.1 kcal mol(-1)). The final data are used to refine the two-body parts of the currently best three-dimensional potential energy surface for this reaction, to predict several spectroscopic parameters of the species involved, and to offer accurate estimates of the title reaction's exothermicity (32.0 kcal mol(-1)) and activation barrier (1.8 kcal mol(-1)) as well as the geometry of the transition state.