Showing papers by "Enrico Clementi published in 1974"

Study of the electronic structure of molecules. XXI. Correlation energy corrections as a functional of the Hartree‐Fock density and its application to the hydrides of the second row atoms

Abstract: A semiempirical functional of the Hartree‐Fock density is presented for estimates of the correlation energy correction. The functional is similar to the one proposed by Gombas and is (a) parametrized with reference to few atomic systems, and (b) is modified as to reproduce the atomic correlation correction not only for closed but also for open shell systems. The functional is then applied to the ground state function of the hydrides LiH(1Σ+), BeH(2Σ+), BH(1Σ+), CH(2Π), NH(3Σ−), OH(2Π), and HF(1Σ+). Several internuclear distances have been considered for each hydride, scanning the potential energy curve from the repulsive region to the dissociation products (∼ 10 a.u.). For these points a simple multiconfigurational function (consisting of no more than three configurations) was computed to obtain Hartree‐Fock functions with proper dissociation behavior (H‐F‐P‐D functions). The semiempirical functional was applied both to the traditional H‐F functions and the H‐F‐P‐D functions in order to study how to selec...

Study of the structure of molecular complexes. VI. Dimers and small clusters of water molecules in the Hartree‐Fock approximation

Abstract: The analytical fit to a large number of Hartree‐Fock computations for the water‐water interaction has been reanalyzed and used to study small clusters of water molecules. With the analytically fitted Hartree‐Fock potential, thousands of possible configurations for the dimers, trimers, tetramers, pentamers, hexamers, heptamers, and octamers of water have been compared in order to determine the configuration of lowest energy (maximal stabilization energy). For the dimer two possible stable configurations are found, corresponding to an open form and a cyclic form, with the open form being more stable. For the trimers and tetramers the cyclic forms are somewhat more stable than the open structures. For the larger clusters it is concluded that it is rather meaningless to consider a single structure, but what is physically relevant is the statistical distribution of different configurations, since many configurations with significantly different geometry have nearly the same energy. The comparison of the stabilization energy per molecule of the different clusters with the corresponding value for liquid water does not support the mixture‐model theories of the structure of liquid water.

Study of the electronic structure of molecules. XXII. Correlation energy corrections as a functional of the Hartree‐Fock type density and its application to the homonuclear diatomic molecules of the second row atoms

Abstract: A semiempirical functional of the Hartree‐Fock type density, previously obtained for the Hartree‐Fock atomic functions and applied with good success to the second row hydrides (in paper XXI of this series), is now applied to the molecules H2(X 1Σg+), Li2(X 1Σg+), Be2(1Σg+), B2(X 3Σg−), C2 (x1 Σg+), N2(X 1Σg+), O2(X 3Σg−), and F2(X 1Σg+). For each molecule the potential energy curve is computed from the repulsive region to large internuclear distances. In order to follow the dissociation to the correct limits, we have added to the Hartree‐Fock configuration of each molecule those configurations which are needed for describing proper dissociation (HFPD functions). It is found that for certain molecules (Li2, B2, C2, and O2) there are configurations which contribute quite significantly in the bonding regions and must be added to the HFPD function to reach a proper reference state function as previously defined (see paper XXI). With our functional of the density applied to the HFPD functions, the calculated d...

Study of the structure of molecular complexes. VIII. Small clusters of water molecules surrounding Li+, Na+, K+, F−, and Cl− ions

Abstract: The two‐body Hartree‐Fock potential for the ion‐water interaction and the two‐body Hartree‐Fock potential for the water‐water interaction have been used in the pairwise additivity approximation to study the Li+(H2O)n, the Na+(H2O)n, the K+(H2O)n, the F−(H2O)n, and the Cl−(H2O)n complexes, where n =2,3,4, ⋯ ,10. The complex configurations have been constrained to have either symmetrical geometries around the central ion or to be free to assume the lowest energy configuration. For n smaller than 5 (depending on the specific ion in consideration), the symmetrical configuration is the lowest energy configuration. For higher values of n, some of the water molecules tend to form a second shell of solvated water around the ion. The configurational optimalization was carried out only at T =0°K; but for a small cluster containing only four molecules of water, calculations have been performed at T =298°K. From the study at 298°K we have computed the correlation functions gI–O, gI–H, gO–O, gO–H, and gH–H (where the ...

Simple basis sets for molecular wavefunctions containing atoms from Z = 2 to Z = 54

Abstract: The double‐zeta orbital exponents for Slater‐type functions have been reoptimized for the ground state functions of the atoms with Z = 2 to the atoms with Z = 36, and accurately computed for the ground state wavefunctions of the atoms with Z = 37 to the atoms with Z = 54 Thus the entire series of basis sets for accurate double‐zeta functions for the atoms from He(1S) to Xe(1S) are now available for molecular computations

Study of the structure of molecular complexes. VII. Effect of correlation energy corrections to the Hartree‐Fock water‐water potential on Monte Carlo simulations of liquid water

Abstract: The two‐body Hartree‐Fock potential for water‐water interactions previously used to describe small water clusters and used in Monte Carlo studies of liquid water has been used to assess the effect of neglecting the correlation energy correction. A number of semiempirical corrections based on the London, Kirkwood, and Wigner formulas were used. They have an energy spread such that they encompass the expected magnitude of the exact correlation energy correction. Short range correlation interactions were included using a modification of the functional of density proposed by Wigner and the corresponding long range interactions were given by a term proportional to r−6 with coefficients due to both London and Kirkwood. After adding the corrections to the Hartree‐Fock potential the various potentials were compared by generating a number of potential energy surfaces. Following this the water‐water potentials were used in a Monte Carlo simulation of liquid water to calculate thermodynamic and structural data. Afte...

A theoretical study of the lithium fluoride molecule in water

Abstract: Monte Carlo results are reported for the structure of molecular clusters consisting of a lithium fluoride ion pair surrounded by 50 water molecules at temperatures of 298 and 500 K using Hartree‐Fock interaction potentials that include some three‐body terms. It is shown that when separated by distances of between 3 and 4 A each ion is surrounded by a nearest‐neighbor shell of four atoms, hydrogen in the case of the fluoride ion and oxygen in the case of the lithium ion. There is evidence of both secondary hydrogen atom and secondary oxygen atom structure around the ions but there is no evidence of any larger hydration shell. It is tentatively concluded that the coordination number of 4 for each ion is responsible for the behavior of lithium and fluoride ions as ``structure makers.''

Ab initio Calculations for the electronic structure of carbazole and trinitrofluorenone

Abstract: The results of the SCF-LCAO-MO calculations for the electronic structure of carbazole and trinitrofluorenone are reported. Comparison is made with previous computations and with experimental data. A qualitative calculation tends to explain satisfactorily the first electro-absorption peak in amorphous films of polyvinylcarbazole and trinitrofluorenone on the basis of a complete charge transfer model. The singlet-triplet splitting predicted for carbazole also compares favorably with the experimental results.

Study of the electronic structure of molecules XXII. Additional ab initio computations for the barrier to internal rotation in polynucleotide chains

Abstract: In a previous paper of this series (paper XIX) we reported an ab initio computation for the barrier to the internal rotation in the sugar‐phosphate‐sugar complex, C10H19O8P; the barrier corresponds to the configuration with ω″ = 45°, ω′ = Ψ′ = Ψ″ = φ′ = 0°, and φ″ varies from 0° to 360°. Here we repeat the same study with the C10H18O8P− complex (where one proton of the phosphate group has been removed). The new potential curve shows essentially a parallel displacement in total energy relative to the previously computed one. In addition we report two new computations of the barriers obtained by varying φ′ from 0° to 360°, while keeping ω″ = 45° and ω′ = Ψ′ = Ψ″ = φ″ = 0° (using Olson and Flory's notation). Again the Hartree‐Fock model predicts similar features for the rotational barriers of the two complexes C10H19O8P and C10H18O8P−. The calculated barrier heights are 4.0, 3.2, and 7.0 kcal/mole for the φ″ variation in the C10H19O8P complex, and 5.7, 3.2, and 8.2 kcal/mole in the C10H18O8P− complex. The barrier heights for the φ′ variation are 18.8 and 30.3 kcal/ mole for C10H19O8P, and 34.8 and 35.2 kcal/mole for C10H18O8P−. Finally, the total energy for the internal rotation of φ″ is decomposed into a sum of contributions, representing either the electronic rearrangement following the rotation for a number of fragments composing the C10H18O8P− complex, or the pairwise interaction of such fragments during the rotation. The location of the potential minima and maxima is predicted by the pairwise interaction of the fragments. The energy for the rearrangement within each fragment is large.

Analytical Hartree‐Fock functions for positive ions from lithium to xenon

Abstract: Analytical wavefunctions in the Hartree‐Fock approximation are computed with Slater type functions for the ground state of the lowest electronic configuration for the first positive ions from Li+ (Z = 3) to Xe+ (Z = 54). Only the computed total energies are reported here and discussed (however, the basis sets, the expansion coefficients, and the orbital energies are made available elsewhere).