Large scale ab initio molecular orbital calculations on
the binding energy of the water dimer have been
performed. These calculations extend the previous correlation
consistent basis set work to include larger
basis sets (up to 574 functions), and core/valence correlation effects
have now been included. The present
work confirms the earlier estimate of −4.9 kcal/mol as the
MP2(FC) basis set limit. Core/valence
correlation
effects are found to increase the binding energy by ∼0.05 kcal/mol.
The best estimate of the electronic
binding energy of the water dimer is −5.0 ± 0.1 kcal/mol.
Correcting this value for zero-point and temperature
effects yields the value ΔH(375) = −3.2 ± 0.1
kcal/mol. This value is within the error limits of the
best
experimental estimate of −3.6 ± 0.5 kcal/mol with the calculations
favoring the lower end of the experimental
energy range. It should be useful to adopt the present estimate in
empirical and semiempirical model potentials.
Ionic clusters comprised of a single alkali metal cation and up to eight water molecules were studied at the Hartree–Fock and correlated levels of theory using the correlation consistent sequence of basis sets. Estimates of the degree of convergence in the computed properties with respect to the complete basis set limit were facilitated by the underlying systematic manner in which the correlation consistent sets approach completeness. In favorable cases, improved property values could be obtained by fitting finite basis set results with a simple analytical expression in order to extrapolate to the complete basis set limit. The sensitivity of structures and binding energies were analyzed with regard to the inclusion of valence and core-valence correlation recovery at the MP2, MP4, and CCSD(T) levels of theory. The replacement of metal core electrons and the introduction of relativistic contributions via effective core potentials was compared to corresponding all-electron results.
10419approximation with two-body correlations has so far been tested only for the propagation of wave functions in model system-bath Hamiltonian~.~ These models used realistic potentials with barriers very similar to those encountered in H-exchange reactions, and the test calculations showed the mean field approximation with explicit system-bath correlation to be extremely accurate over times long enough for the wave packet to move away from the saddle point region. Nevertheless, numerical calculations of rate constants for problems with three or more degrees of freedom must be carried out to verify the applicability of the approach presented here to more challenging problems. Some such applications are in progress.
Acknowledgment. This work has been supported by a JuniorFellowship from the Society of Fellows, Harvard University. The calculations reported were performed on a S U N 4/65 SPARC 1 station, funded by the Milton Fund Award of the Harvard Medical School.Ab initio electronic structure calculations using analytical energy derivative methods and automated potential energy surface walking techniques have been carried out on the tautomerization reaction path connecting formamide (F) H2N-CH0, through a transition state (TS), to formamidic acid (FA) HN-CHOH. The zero-point corrected F -FA, and F -TS energy differences are predicted to be 12.1 and 48.9 kcal/mol, respectively, when configuration interaction methods are used to treat electron correlation. An imaginary frequency of 23911' cm-' is obtained along the reaction coordinate at the TS. Isotopic substitution of F to generate H,N-CDO and subsequent calculation of the harmonic vibrational frequencies and eigenvectors allowed ambiguities in the assignment of the infrared spectrum of F to be resolved. The geometry of the F tautomer is found to be slightly nonplanar, but to have zero-point energy that permits the planar geometry to be dynamically accessed. Extensions to situations in which tautomerization is assisted by neighboring solvent molecule(s) are considered. In particular, the intimate involvement of a single H 2 0 solvent molecule reduces the zero-point-corrected F -FA and F -TS energy differences to 10.6 and 22.6 kcal/mol, respectively. Intimate solvent participation is thus found to much more strongly affect the activation energy than the overall thermodynamics in this case. The imaginary frequency corresponding to the reaction coordinate at the transition state changes to 20011' cm-I when a single H 2 0 is intimately involved.
Three hydrogen-bonded minima on the phenol-water, C6H50H-H20, potential energy surface were located with 3-21G and 6-31G** basis sets at both Hartree-Fock and MP2 levels of theory. MP2 binding energies were computed using large "correlation consistent" basis sets that included extra diffuse functions on all atoms. An estimate of the effect of expanding the basis set to the triple-zeta level (multiple f functions on carbon and oxygen and multiple d functions on hydrogen) was derived from calculations on a related prototype system. The best estimates of the electronic binding energies for the three minima are -7.8, -5.0, and -2.0 kcalimol. The consequences of uncertainties in the geometries and limitations in the level of correlation recovery are analyzed. It is suggested that our best estimates will likely underestimate the complete basis set, full C1 values by 0.1-0.3 kcal/mol. Vibrational normal modes were determined for all three minima, including an MP2/6-31G** analysis for the most strongly bound complex. Computational strategies for larger phenol-water complexes are discussed.
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