Showing papers by "Enrico Clementi published in 1973"

Study of the structure of molecular complexes. IV. The Hartree‐Fock potential for the water dimer and its application to the liquid state

Abstract: The Hartree‐Fock energy of the water dimer has been computed for 216 different nuclear configurations using the basis set given in the first paper of this series. Near the equilibrium configuration, a calculation was carried out using a large Gaussian basis set with optimized orbital exponents for the oxygen 3d ‐ and 4f ‐type and hydrogen 2p ‐ and 3d ‐type functions in order to get results close to the Hartree‐Fock limit. In the vicinity of the equilibrium configuration the mechanism for binding of the dimer is analyzed with the help of the bond energy analysis formalism. The importance of polarization (internal charge transfer), as pointed out previously by a number of authors, is clearly evident. The computed energies have been used to derive a simple analytical expression that reproduces the Hartree‐Fock potential energy surface to a high degree of accuracy. This analytical potential is compared with the empirical effective pair potentials proposed by Rowlinson and Ben‐Naim and Stillinger for the descr...

Study of the structure of molecular complexes. V. Heat of formation for the Li+, Na+, K+, F−, and Cl− ion complexes with a single water molecule

Abstract: In order to obtain the heat of formation ΔH, for the ion‐water complexes previously studied in the Hartree‐Fock approximation in the first three papers of this series, we have computed the normal frequencies of the complexes, the zero‐point energy correction to ΔH, and the molecular extra correlation energy. The main contribution to ΔH is due to the Hartree‐Fock binding; the least important contribution results from the correlation effects. The Hartree‐Fock binding varies from about 35 kcal/mole (Li+–H2O) to about 12 kcal/mole (Cl−–H2O); the zero‐point correction is between 1 and 2 kcal/mole; and the molecular extra correlation correction is less than 1 kcal/mole. The computation of ΔH is analyzed in order to estimate upper and lower bounds. We conclude that the calculated ΔH values are accurate to about 2.0 kcal/mole. Experimental data support this conclusion. In the Appendix, the potentials for water‐ion complexes have been presented in the form of a simple analytical expansion. The expansion has been obtained by fitting the Hartree‐Fock computed energies for the water‐ion complexes.

Study of the structure of molecular complexes. II. Energy surfaces for a water molecule in the field of a sodium or potassium cation

Abstract: The previously reported study of the energy surface of a water molecule in the field of an Li+ ion is extended to the surfaces of water in the field of an Na+ or K+ ion. The computations are done in the Hartree‐Fock approximation using a large basis set of Gaussian functions. Calculated binding energies and cation‐H2O bond distances in the most stable configuration are 35.2, 25.2, 17.5 kcal/mole and 3.58, 4.25, 5.08 a.u. for Li+, Na+, and K+, respectively. A detailed analysis and comparison of the potential energy surfaces are reported by considering a number of cross sections through the surfaces and by decomposing the total energy of the complex into the sum of the energy of H2O in the field of the cation, the energy of the cation in the field of H2O, and the cation‐H2O interaction energy.

Study of the structure of molecular complexes. III. Energy surface of a water molecule in the field of a fluorine or chlorine anion

Abstract: The interaction between F− or the Cl− anion and water has been computed in the Hartree‐Fock approximation with a large basis set of Gaussian functions: The obtained energies and wavefunctions are therefore close to the Hartree‐Fock limit. We find that the geometrical configuration of maximal stability corresponds to a hydrogen‐bonded configuration between the water and the anion; however, the hydrogen bond is somewhat bent with a deviation from linearity of about 4.5° for F−–H2O and about 14.6° for Cl−–H2O. At the most stable configuration the oxygen‐anion distance is 4.75 a.u. for F− and 6.26 a.u. for Cl−. At the equilibrium configuration the Hartree‐Fock binding energy is 23.54 and 11.86 kcal/mole for the F−–H2O and the Cl−–H2O complex, respectively. The energy surfaces are analyzed in terms of an energy partitioning consisting of the energy of the water molecule in the field of the anion, the anion in the field of the molecule of water, and of the interaction energy between the water molecule and the a...

Study of the electronic structure of molecules. XVIII. Interaction between a lithium atom and a cyano group as an example of a polytopic bond

Abstract: We have reinvestigated the LiCN and LiNC molecules in the linear configurations. With very near to Hartree‐Fock functions we obtain the following equilibrium distances: R(Li–C)=3.65± 0.015 a.u., R(C–N)=2.14± 0.015 a.u. for LiCN; R(Li–N)=3.35± 0.015 a.u. and R(C–N)=2.18± 0.015 a.u. for LiNC. The Hartree‐Fock energy (at equilibrium) are −99.8176 a.u. for LiCN and −99.8275 a.u. for LiNC. These data are corrected using a statistical model to compute the correlation energy. The resulting atomization energy (total binding relative to dissociation into ground state atoms) is 13.48 eV for LiCN and 13.78 eV for LiNC. (These data are, therefore, confirming previous computation by Bak, Clementi, and Kortzeborn, done with smaller basis set.) With a constant value for the C–N distance, we have mapped the energy surface for a number of geometries in nonlinear conformations, following the lowest energy path leading from LiCN to LiNC: no energy barrier was found between the LiCN and the LiNC linear configuration or betwe...