Accurate Thermochemistry Research Articles

Ab initio determination of the structure of the active site of a metalloenzyme: Metal substitution in phosphotriesterase using density functional methods

A number of previous studies have determined that a metalloenzyme active site is inherently determined by the ligands and metal and only weakly perturbed by the surrounding protein environment. This conclusion is now being tested for several families of bimetallic enzymes. Metal substitution is examined for phosphotriesterase (PTE) with all possible substitutions of zinc and cadmium in the two sites. A new density functional theory (DFT) functional is used in this study which has yielded accurate structures and thermochemistry for molecules made up of first and second row atoms. Comparisons between gradient-optimized Hartree–Fock and DFT structures are in good agreement, suggesting this functional is useful in studying transition metal enzyme active sites. Good agreement between the X-ray and in vacuo optimized structures is also obtained for the Zn–Zn and Cd–Cd enzymes. This supports the analysis of the PTE active site as an inherent complex and suggests that accurate structures can be obtained for other metal substitutions for which no experimental structures are available. A unique zinc/cadmium hybrid was observed experimentally, and the structure of this active site is theoretically predicted. The polarization of bound water at this very polar active site is very large, suggesting it is the reactive nucleophile. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 289–299, 1999

Accurate density functional thermochemistry for larger molecules

Density functional methods are combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. Seven different density functionals are assessed for the evaluation of heats of formation, Delta H 0 (298 K), for a test set of 40 molecules composed of H, C, O and N. The use of bond separation energies results in a dramatic improvement in the accuracy of all the density functionals. The B3-LYP functional has the smallest mean absolute deviation from experiment (1.5 kcal mol/f).

Accurate density functional thermochemistry for larger molecules

Density functional methods are combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. Seven different density functionals are assessed for the evaluation of heats of formation, Δ H f 0 (298 K), for a test set of 40 molecules composed of H, C, O and N. The use of bond separation energies results in a dramatic improvement in the accuracy of all the density functionals. The B3-LYP functional has the smallest mean absolute deviation from experiment (1·5 kcal mol f -1).

Accurate thermochemistry for larger molecules: Gaussian-2 theory with bond separation energies

Gaussian-2 (G2) theory is combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. For a test set of 40 molecules composed of H, C, O, and N, our method yields enthalpies of formation, ΔHf0(298 K), with a mean absolute deviation from experiment of only 0.5 kcal/mol. This is an improvement of a factor of three over the deviation of 1.5 kcal/mol seen in standard G2 theory.

Accurate ab Initio Quartic Force Fields and Thermochemistry of FNO and ClNO

Accurate ab Initio Quartic Force Fields and Thermochemistry of FNO and ClNO

Density-functional thermochemistry. III. The role of exact exchange

Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.