Nonempirical potential

Nonempirical potential-energy curves have been calculated for a number of heteronuclear diatomic molecules. Hartree-Fock curves include a number of unpublished ones from IBM [56-58], which are listed in the NBS report [33] among them LiF, LiCl, BeO, BF, CO, NaF, NaCl, MgO, A1F, SiO, PN, CaO, SrO, and others, which were also mentioned at the recent Sanibel... 

Curves that go beyond the Flartree-Fock method have been calculated for certain systems, and the list grows monthly. Of special interest is the series on ground [62-64] and excited states [63, 64] of CO, on NaLi and NaLi+ using an extended Flartree-Fock method with optimized double-valence configurations [65], HeLi by a valence-bond method [66], and so on. A quite complete listing of all nonempirical potential-energy curves calculated through 1967 is included in the NBS report [33]. 

The use of nonempirical potentials in this way " in Monte Carlo calculations was pioneered and systematically developed " by dementi and his group. Since the applications and results obtained were re-viewed recently, we note only briefly on some of them. A critical test for this theoretical approach was provided by the treatments on liquid water, In the cited papers the calculated ra-... 

It should represent the true potential accurately in the interaction regions for which experimental or nonempirical theoretical data are available. 

Briggs For unstable and neutral conditions Ah = 0.25Qh" hp U For stable conditions Ah = 0.296[Q /U(ae/az] where 56/5z = variation of potential temperature with height = 0.03 K/m Nonempirical formulation... 

Photoelectron spectra have been reported for 2,4 and A-methylisoindole ° and the ionization potentials (IP) assigned in the light of nonempirical calculations using Koopmans theorem. Linear correlations of the type IPobs = I Peak + b were obtained in all three cases. As was noted, extended Hiickel, PPP, and other semiempirical calculations also led to satisfactory correlations of the first three IPs, but the scatter was generally larger. The first IP of 4 lies in the order of 7.9 eV (Fig. 1 of Palmer and Kennedy ) a value of 7.91 eV has been reported by other authors. In comparison, the first IP of 1,3-diphenylbenzo[c]furan is 7.09 eV. ... 

The interaction parameters for the water molecules were taken from nonempirical configuration interaction calculations for water dimers (41) that have been shown to give good agreement between experimental radial distribution functions and simulations at low sorbate densities. The potential terms for the water-ferrierite interaction consisted of repulsion, dispersion, and electrostatic terms. The first two of these terms are the components of the 6-12 Lennard-Jones function, and the electrostatic term accounts for long-range contributions and is evaluated by an Ewald summation. The... 

Nonempirical LCAO-MO-SCP study for linear HeH surface demonstrates a late barrier, or potential-energy step characteristic for reactions showing vibrational enhancement (Fig. 61) 453... 

J. Sponer, J. Leszczynski, P. Hobza, Nature of nucleic acid-base stacking Nonempirical ab initio and empirical potential characterization of 10 stacked base dimers. Comparison of stacked and H-bonded base pairs. J. Phys. Chem. 100, 5590-5596 (1996)... 

Hobza P, Kabelac M, Sponer J, Mejzlik P, Vondrasek J (1997) Performance of empirical potentials (AMBER, CFF95, CVFF, CHARMM, OPS, POLTEV), semiemprical quantum chemical methods (AMI, MNDO/M, PM3) and ab initio Hartree-Fock method for interaction of DNA bases comparison of nonempirical beyond Hartree-Fock results, J Comp Chem, 18 1136-1150... 

Some references to particularly pertinent molecular-orbital or configuration-interaction calculations of potential curves of diatomic molecules will be mentioned below in this section. These references are not intended to be exhaustive but should serve to indicate some of the calculations which have been carried out and the type of information which is available. (The most comprehensive compilation of nonempirical molecular-orbital calculations is a 1967 survey by Krauss [33], which includes both diatomic and polyatomic molecules.)... 

Having developed and parameterized potential models, the final stage before their use in a simulation study should be their evaluation. Nonempirically derived potentials should be evaluated by reference to their ability to predict empirical crystal properties. For empirical potentials, it is clearly necessary to use data outside the range employed in the parameterization. We have already referred to the use of lattice dynamical data. Comparison with the results of high-pressure studies, in particular the variation of structural and elastic properties with pressure, is also of great value and... 

P.-O Astrand, A. Wallqvist and G. Kalstrdm, Nonempirical intermolecular potentials for urea-water systems, J. Chem. Phys., 100 (1994) 1262-1273. 

W. A. Sokalski, A. H. Lowrey, S. Roszak, V. Lewchenko, J. Blaisdell, P. C. Hariharan, and J. J. Kaufman,/. Comput. Chem., 7,693 (1986). Nonempirical Atom-Atom Potentials for Main Components of Intermolecular Interaction Energy. 

Quantum-mechanical studies on the tautomerism of heterocyclic compounds involve, in general, two aspects. The first deals with the prediction of physicochemical properties of defined tautomeric forms (e.g., ultraviolet spectra, dipole moments, ionization potentials, etc.). This seems to be easy to handle. Using any semiempirical or nonempirical quantum-mechanical computational method, depending on approximations involved in the method, we are able to calculate properties that, more or less, agree with experimental values. Calculations of this type do not contribute to a direct estimation of the relative stability of the tautomers, however they are particularly important for cases in which a tautomeric form of a compound is so rare that it is not possible to measure it directly. 

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