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A significant effort has teen made in order to improve the continuum model used for calculation of the solvation thermodynamic quantities of a molecule embedded in a cavity formed by the intersecting van der Waals spheres of the solute in a polarizable medium. These improvements principally concern the thermodynamic quantities associated with the electrostatic part of the solvation energy. A simple method is proposed for the calculation of the fictive charge density representing the reaction potential. The calculated values obtained agree quite well with those calculated within more sophisticated methods. The improvements also principally concern the representation of solvation sites. The method is applied to the calculation of the vaporization thermodynamic quantities of nonassociated liquids, and the results obtained are discussed in relation with experimental data.

structure, led Srivastava19 to assign a planar structure to anthrone. Therefore, the enthalpy of formation (Table III) and resonance energy (Table IV) of anthrone were estimated and compared with those of xanthone. The resonance stabilization energy of xanthone falls between those of anthraquinone and anthrone, both of which are known to be planar molecules. These findings suggest that xanthone may also be planar. The somewhat greater resonance energy of xanthone, when compared with that of anthrone, may be due to the contribution of the unshared electrons of the ether oxygen atom to the resonance conjugation. This is consistent with the reported results20-22 that the xanthone molecules, to which the 7-pyrone ring is coupled, are aromatic.

Some aspects of the use of simplified formulas for the evaluation the interaction energy between two molecules is discussed. This energy is obtained as the sum of four terms: electrostatic, polarization, dispersion, and short-range (exponentially decreasing) repulsion. The effect of using several approximations is considered (i) for representing the charge distributions of the isolated molecules (which determine the electrostatic and polarization terms), and (ii) for evaluating the short-range repulsion term. The various charge distributions considered are semiempirical atomic net charges (Del Re + Pariser-Parr), atomic charges and dipoles (CNDO) and corresponding "effective" atomic charges, and many-centered multipole distributions obtained from ab initio SCF calculations (charges, dipoles, and quadrupoles located on the atoms and the middles of segments joining pairs of atoms, whether chemically bonded or not). As concerns the short-range repulsion, the various procedures considered are a sum of atom-atom terms, a sum of bond-bond terms, and the use of anisotropic van der Waals radii for heteroatoms (oxygen and nitrogen). In all cases, the dispersion energy is obtained as a sum of atom-atom terms (of the R-6 type). These various procedures are checked in the case of the interactions between nucleic acid bases, for two rather different kinds of configurations, namely, hydrogen bonded and stacked. This comparison reveals a rather complicated picture, namely the results got from the various levels of approximation of the molecular charge distribution lead to different degrees of agreement according to different situations, e.g., for guanine-cytosine interactions, qualitative agreement is found between the various methods as concerns the relative order of stacked and hydrogen bonded situations, while agreement is much less satisfactory for adenine-uracile interactions. One of the main conclusions is that a sufficiently sophisticated representation of the molecular charge distribution is required in order to get reliable results for all possible configurations of a complex. Such comparative studies should bring significant help as concerns the development of the reliable simplified procedures for evaluating intermolecular interaction energies.

Articles you may be interested inIntermolecular vibrational modes and orientational dynamics of cooperative hydrogen-bonding dimer of 7azaindole in solution Comment on "Anisotropic intermolecular interactions in van der Waals and hydrogen-bonded complexes: What can we get from density-functional calculations?" [J.In this work we test two ab initio methodologies which allow the decomposition of the total intermolecular interaction energy into physically meaningful contributions, namely, the symmetry adapted perturbation theory ͑SAPT͒ and the use of localized orbitals within a Møller-Plesset perturbation scheme. The accuracy of the two different methods is compared to supermolecular results, within MP2 and coupled-cluster theory within single and double excitations, with perturbative estimates of the amplitudes of triple excitations ͓CCSD͑T͔͒. Some relations between the different approaches are conjectured from theoretical considerations, and are confirmed by numerical results. The corresponding calculations have been performed for three model dimers: two NH 3¯H2 O dimers, with NH 3 acting once as a proton acceptor and once as a proton donor, and the NH 4 ϩ¯H 2 O considered as a prototype of the ion-molecule interaction. We may conclude that third-order terms in SAPT help significantly to reproduce the Hartree-Fock induction and the relaxed, total dispersion in the LMP2 decomposition.

An integrated procedure that computes in a consistent way both the intermolecular interaction energies and the solvation energies is reported. It interfaces the S u m of Interactions Between Fragments Ab initio computed molecular mechanics and the Langlet

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