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By using the theory of intermolecular forces, two new expressions for Pauli repulsion and dispersion contributions to the solvation free energy are derived. These expressions contain explicitly the solute electron density and, therefore, can be used directly in the SCF calculation of the solute wave function within the polarizable continuum model (PCM). The final expressions are very simple and include also some intrinsic solvent properties which are, for repulsion, the density, the molecular weight, the number of valence electrons, and for dispersion, the refractive index and the ionization potential. This new approach does not depend on any given intermolecular potential and it can be adapted to any choice of basis set. For small-size basis sets, even minimal, the dispersion contribution is obtained in two steps and includes the effect of adding diffuse and polarization functions, not used in the wave function itself. This method has been implemented in our HONDO package, in a version which includes the cavitation contribution, determined by the Pierotti−Claverie method, and the polarization contribution determined by the Miertus−Scrocco−Tomasi method. Some preliminary results on solutes containing C, H, O, and N are presented for solvation in water, n-hexane, and 1-octanol. The quality of these results, given the simplicity of the PCM, is acceptable and of great interest for future developments.

We present a method for computing intermolecular energies of large molecules based on a suitable fragmentation scheme, which allows one to express the complete interaction energy as a sum of interaction energies between pairs of fragments. The main advantage consists in the possibility of using standard ab initio quantum methods to evaluate the fragment energies. For the 4-n-pentyl-4′-cyanobiphenyl (5CB) dimer, the present results indicate that the most favorite arrangement corresponds to an antiparallel side-by-side geometry with a stabilization energy of about 16 kcal/mol. It is shown that, by the present method, the interaction energy of the 5CB dimer can be evaluated for all geometrical conformations and, in principle, it can be used for bulk simulations.

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The Menshutkin reaction NH3 + CH3 Cl → CH3NH3 + + Cl- in aqueous solution is studied using the CASSCF method combined with the polarizable continuum model in a version that includes electrostatic, repulsion, and dispersion solute−solvent interactions. The solvent reaction field is inserted in the CASSCF Hamiltonian resorting to a mean-field approximation. The C 3 v symmetry is maintained for all the geometries considered and the active space is generated distributing four electrons in four orbitals of a1 type. The results of the present study are in excellent agreement with recent ab initio calculations, which use both continuum and discrete solvation models, and with available experimental data. The analysis of the electronic structure of the transition state by the VB method explains the mechanism of this reaction in terms of a Linnett-type nonpaired spatial orbital representation as in a four-electron, three-center bonding unit.

We propose a new class of multideterminantal Jastrow-Slater wave functions constructed with localized orbitals and designed to describe complex potential energy surfaces of molecular systems for use in quantum Monte Carlo (QMC). Inspired by the generalized valence bond formalism, we elaborate a coupling scheme between electron pairs which progressively includes new classes of excitations in the determinantal component of the wave function. In this scheme, we exploit the local nature of the orbitals to construct wave functions which have increasing complexity but scale linearly. The resulting wave functions are compact, can correlate all valence electrons, and are size extensive. We assess the performance of our wave functions in QMC calculations of the homolytic fragmentation of N-N, N-O, C-O, and C-N bonds, very common in molecules of biological interest. We find excellent agreement with experiments, and, even with the simplest forms of our wave functions, we satisfy chemical accuracy and obtain dissociation energies of equivalent quality to the CCSD(T) results computed with the large cc-pV5Z basis set.

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