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Inspired by the helical structure as well as the conductivity of DNA and substituted polyphenylacetylenes, a new kind of substituted polyacetylene is introduced. More specifically, polycytosineacetylene ((RC CH) n , R = cytosine) with a trans-polyacetylene backbone is investigated theoretically. The neutral and cationic geometries are optimized as single helical chains with n = 20. It is found that when an electron is removed the polymer stretches from 19.80 A to 28.41 A. The polyacetylene backbone in the optimized cationic polymer exhibits the formation of a soliton. It is predicted that a doped polycytosineacetylene would have good conductivity, and would also behave as a nanomechanical spring and as a piezoelectric material.

We propose a general definition of the radius of an atom in terms of its ionization energy. A relationship between these two fundamental properties is derived from the radial distribution function and the exponential decay of the electron density. Strong correlations with well-known atomic radii lead us to conclude that we have found a universally valid definition of the atomic radius, defined exclusively in terms of an intrinsic property, the ionization energy of an atom, which is equally available from experiments and theoretical computations.

Triply hydrogen-bonded complexes of the form AAA-DDD are shown to have the strongest interaction when the complex is substituted with electron withdrawing groups on the donor molecule (DDD) and electron donating groups on the acceptor molecule (AAA). In particular, the largest effects are observed when the withdrawing groups act through resonance. This serves to flatten the entire system resulting in more linear, and consequently stronger, hydrogen bonds. Furthermore, the present calculations show that the binding energy correlates with the electron density at the bond critical points and inversely with the hydrogen bond lengths.

We report a study of the inclusion complex of p-chlorophenol inside α-cyclodextrin (α-CD) by the theory of atoms in molecules (AIM). We use a quantitative comparison of some AIM properties of isolated p-chlorophenol (PCP) and the inclusion complex (PCP-CD) and we characterize some weak interactions within the host–guest complex. Furthermore, we compare the electrophilic aromatic substitution on the p-chlorophenol in the isolated state and inside α-CD. The analysis of the bond critical points of PCP shows that there is no trend in the effect on the AIM properties of PCP due to inclusion in the α-CD.

A type of cage critical point (CCP) that has remained so far only a ‘mathematical possibility’ is reported for the first time. A CCP enclosed by two ring surfaces is found in a single ring in each of three derivatives of 1,12-difluorobenzo[c]phenanthrene. The two fluorine nuclei are linked by a bond path which results in the closure of a seven-membered ring in the ‘bay’ region. The crowding of the fluorine atoms forces an α-helical (staircase) twisting of the molecule causing the ring surface to split into two giving rise to a CCP. The Poincare–Hopf relationship is satisfied in each case.

We report a detailed characterization of the valence shell charge concentration (VSCC) of fluorine in methyl fluoride through the atoms in molecules theory. A comparison between the VSCCs of fluorine and neighboring elements (N, O, P, S, Cl) in the periodic table is also presented. The fluorine VSCC is shown to be tetrahedral and the most compact with the highest values of ρ and −∇ 2 ρ . Our results calculated at several post Hartree–Fock levels of theory using a large basis set show a remarkable insensitivity to the level of theory.

The performance of the DFT functionals B3LYP, B3PW91 and PBE1PBE in conjunction with selected basis sets for the prediction of 15N and 13C nuclear shieldings for small nitrogen-containing molecules is assessed. For molecules containing only singly bonded nitrogen and carbon atoms, the PBE1PBE/6-311+G(2df,p) level of theory is shown to provide reliable 15N and 13C nuclear shieldings. For the triply bonded nitrogen-containing molecules N2, CH3CN and HCN, all three levels of theory are much less satisfactory.

Leukotrienes (LT) are a family of drug-like molecules involved in the pathobiology of bronchial asthma and are responsible for smooth muscle contraction. Leukotriene C4 synthase (LTC4S) is a nuclear-membrane enzyme responsible for the conjugation of leukotriene A4 (LTA4) to glutathione to form LTC4, a cysteinyl leukotriene. In this study, the mechanism of LTA4 binding by LTC4S has been computationally examined. More specifically, docking and molecular dynamics simulations were used to gain insight into the substrate-bound active site. These studies identified two possible orientations for bound LTA4: ‘tail-to-head’ and ‘head-to-tail’. An ONIOM(QM/MM) approach was then used to elucidate the mechanism by which glutathione may add to LTA4. In particular, the thiolate of glutathione acts as a nucleophile attacking C6 of LTA4 forming a S–C6 bond. Concomitantly, a proton is transferred from the guanidinium of Arg31 to the epoxide ring oxygen. This results in opening of the epoxide ring and stabilization of the LTC4 product complex. Within the present computational methodology the ‘tail-to-head’ orientation appears to be the most likely substrate orientation.

Herein we present an assessment to determine which of nine well-established DFT functionals best describes the reduction of C2H2Se2(-)˙. In addition, we have also studied the effects of changing the substituents bound to the alkene functional group of dithiolene and diselenolene ligands. Such ligands are important due to their unique electrochemical and physical properties when ligated to metals. The M06-L functional shows best agreement with the QCISD/cc-pVTZ value of -2.45 V for the reduction potential of the (C2H2Se2˙(-)/C2H2Se2(-2)) redox couple. At the M06-L/6-311+G(d,p) level of theory the calculated reduction potential for the (C2H2Se2˙(-)/C2H2Se2(-2)) redox couple is only 0.09 V in error. However, as a result of the nature of the oxidized species for the respective ligands the absolute reduction potential of the (C2H2Se2˙(-)/C2H2Se2(-2)) redox couple is 0.57 V more oxidizing than the (C2H2S2˙(-)/C2H2S2(-2)) redox couple. This is due to the radical electron in C2H2S2˙(-) being delocalized within the alkene backbone, whereas in C2H2Se2˙(-) the electron is largely localized on the Se atoms. The relative reducing power of the S- and Se-containing redox couples is shown to vary depending on the choice of substituents. In particular the reduction potential of the various S-containing redox couples range from being 0.34 V more reducing to 0.28 V more oxidizing than the analogous Se-containing redox couples. This difference in the relative reducing power appears to be a result of the nature of the oxidized ligand. Thus, depending on the choice of moiety very different chemistry is seen between the analogous dithiolate and diselenolate ligands.

The gas phase and solution electron affinities and ionization potentials of uracil, thymine and a series of 5-halouracils ( 5XU, X=F, Cl, Br ) are investigated with B3LYP. Halogen substitution has a smaller effect on the IP than the EA of U . The EAs are calculated to increase according to T U 5FU 5ClU 5BrU . The calculated barriers for the dissociation of the resulting 5XU anions to X − plus uracil-centered radicals decrease along the series 5FU − > 5ClU − > 5BrU − . The calculated trends are consistent with suggestions that 5XUs enhance the sensitivity of deoxyribonucleic acid, DNA (ribonucleic acid, RNA) to ionizing radiation and that 5BrU leads to the greatest enhancement.

The structure sensitivity and cluster size convergence for formate adsorption on the Cu(100), Cu(110) and Cu(111) surfaces have been investigated systematically using density functional theory and the cluster model containing up to 40 Cu atoms. The copper core–valence correlation effect on the adsorbate–surface interaction is examined by using three different basis sets and effective core potentials. The calculated geometries and vibrational frequencies are in good agreement with experimental data even on the small clusters and are not surface sensitive. However, the adsorption energies show strong dependence on the surface structure and the cluster size. The adsorption energies are shown to converge very well for the large clusters, and the activity of the Cu planes for formate adsorption is in the order of Cu(110)>Cu(100)>Cu(111), the same as that observed experimentally for methanol synthesis. Regardless of the basis set, cluster size and surface structure, all results show an anionic formate adsorption species. The chemisorption mechanism and the local structure of formate on the three copper surfaces are essentially very similar. Some discussion about cluster modeling is presented.

Density-functional theory (B3LYP functional) is used to investigate the ionization potentials and electron affinities of the DNA and RNA nucleotide bases. For the first time, anions lying lower in energy than the neutral species have been calculated for both thymine and uracil (i.e., positive adiabatic electron affinities). Additionally, the calculations show that anion formation leads to significant geometrical changes to the nucleobases. This is a very important finding as previous calculations have indicated that the anions are very similar in geometry to the neutral species and reported negative valence adiabatic electron affinities.

The radial moments of the electron density, 〈rn〉, have been calculated for N2 and H2O at the quadratic configuration interaction (with singles and doubles) levels of theory by use of a variety of basis sets. An optimal basis set was chosen from the first set of calculations and then used with several conventional ab initio and density functional methods to compute the moments for a representative set of molecules: N2, CO, CH4, NH3, H2O, HF, SiH4, PH3, H2S, HCl, CH3OH, and CH3SH. The effects of solvation on the radial moments were also studied using the Onsager model and an extension of the Tomasi polarized continuum model.

Temperature, vibrational, and matrix effects on the geometry and hyperfine coupling constants of the methane and ethane radical cations are investigated with a combined quantum mechanics and molecu ...

The hyperfine coupling constants (HFCCs) in the hydroxyl radical are investigated through comparison of results obtained from a variety of quantum chemical methods. The couplings obtained from the multi-reference configuration interaction (MRCI) wave function, built upon the restricted open-shell Hartree–Fock (ROHF) reference determinant, are investigated in terms of the basis set, the configuration selection energy threshold, and the size of the reference space. Overall results which converge to the experimental couplings are obtained for hydrogen, but not for oxygen. In particular, the MRCI method shows no improvement over density functional theory (the B3LYP functional), for the calculation of Aiso(17O). On the other hand, results in excellent agreement with experiment are obtained through the use of the quadratic configuration interaction (QCISD) method based on the unrestricted HF (UHF) reference determinant with the identical basis sets. Examination of UHF and ROHF based coupled-cluster methods, CCS...

By analogy with the Carbo overlaplike similarity measure, a spin-density polarization index (SPI) is defined. This index gives the Euclidean distance between the α and β electron densities for an open-shell molecule. An extensive study of the O2 molecule reveals that the SPI is highly dependent on the theoretical method, but is less sensitive to the basis set provided flexible extended basis sets are used. The radial distribution of the O2 spherically averaged electron density also reveals that near the center of the bond there is an inversion of the α and β electron densities and that there is a minimum in the β electron density near the nuclei within the internuclear region. Finally, the SPI is calculated for a variety of small molecules and the results are compared with those obtained for O2.

A new semiquantitative measure of the difference between two electron density distributions is proposed. Specifically, the natural orbitals of an accurate electron density, such as those given by a configuration interaction method, are used to expand the Kohn–Sham orbitals of an approximate density functional method. The difference between the two density distributions is then reduced to a single number, the orbital-based density difference index (DDI). With the reference densities calculated from quadratic configuration interaction calculations including single and double substitutions, DDIs were obtained for four diatomic molecules using three basis sets and seven approximate functionals. Results are also included for an additional six small molecules with a flexible extended basis set and the same set of functionals. These results show that the DDI leads to conclusions that are consistent with those obtained previously by visual comparison of density difference plots. The orbital-based density differen...

The convergence of basis set contractions is examined at the multireference configuration interaction level with respect to the 14N isotropic hyperfine coupling constant in NH2, a case that is very sensitive to the choice of the basis set and the degree of correlation recovery. The contraction schemes studied include the atomic natural orbital (ANO) approach and two Hartree–Fock-based contraction methods: Raffenetti’s general contraction method and the segmented method. Contractions of a (13s8p2d) primitive set ranging from [4s3p2d] to [8s3p2d] in the s space and from [7s3p2d] to [7s6p2d] in the p space are employed. It is found that the ANO contraction yields the smoothest and fastest convergence, although all three contractions converge to the uncontracted results. Thus, the ANO contraction starts to converge at [6s3p2d] while the other two converge at [7s4p2d]. The correlation recovery of the core electrons versus that of the valence electrons is also studied. In contrast to previous speculation, it is...

The molecular quadrupole moments of benzene and naphthalene are calculated using a range of ab initio and density functional methods, with basis sets as large as aug-CC-PVTZ for benzene and CC-PVTZ for naphthalene. The quadrupole moments calculated using Hartree-Fock wavefunctions are lower than the experimentally determined values, and all DFT values are higher than the experimental values. The use of the hybrid B3P86 and B3PW91 functionals, with a large basis set, produce quadrupole moments which are within experimental error for these two small aromatic molecules.

The structures and relative stabilities of the four different forms of hydroxypyridine and hydroxyquinoline (normal, protonated cation, deprotonated anion and zwitterion) in the gas phase have been calculated at the Moller-Plesset (MP) perturbation and coupled-cluster theory levels with basis sets up to 6–31+ G ∗∗ . The solvation free energies of the reactions were treated with the SMx series of aqueous solvation models by Cramer and Truhlar. By combining the PM3-SM3 model for solvation with the MP 2/6–31+ G ∗∗ level treatment for the gass-phase species, it is found that the free energy changes of the proton transfer reactions can be predicted reasonably well.

A hybrid quantum mechanical molecular dynamics method is used to study liquid methanol at room temperature and normal density. Frequencies of the twelve vibrational modes are calculated from the simulation data at the ab initio Hartree–Fock/3‐21G(d,p) level. Good overall agreement is found between the experimental and calculated frequencies. Three different, successive levels of quantum mechanical/molecular mechanical (QM/MM) coupling schemes are investigated using gas‐to‐liquid vibrational frequency shifts as a probe. The results suggest, somewhat surprisingly, that the method with the weakest QM/MM coupling gives the best overall agreement between the experimental and simulated results for vibrational frequency shifts. The most elaborate coupling scheme overestimates the shifts towards the red direction due to overestimation of the attractive interactions between quantum mechanical and molecular mechanical molecules, while it is found to be most successful in describing the O–H stretch. The effects of the solvent on the geometrical parameters of methanol are investigated in detail.

The thermally induced decomposition of ethylene, C2H4, to acetylene, C2H2, on the Ni(111) surface has been treated theoretically: potential-energy curves were computed within a modified extended-Huckel framework and reaction rates estimated via a master equation approach. As a preliminary step, adsorbate geometries and binding energies were optimized for one and two molecules of each hydrocarbon species chemisorbed in various sites on rigid bulk cluster models of the Ni substrate. A fragmentation pathway is proposed, eliciting the concerted tunnelling of intermolecular H's from a transient bimolecular complex comprised of two rotated C2H4's, which rationalizes the observed second-order decomposition kinetics.

The 2Σ+ states of HBeO, HMgO, and HCaO have been studied at the CASSCF level with a basis set of at least triple‐zeta plus double polarization quality. The correlation treatment includes the nine valence electrons and ten orbitals resulting in an active space denoted by (622). The optimized structures of HMO (M=Be, Mg, and Ca) are all linear and the energy of HMO lies around 250 kJ/mol (2.6 eV) above that of MOH. The M–O bond length in HMO is predicted to be longer than that in the corresponding MOH by amounts ranging from 0.01 A to 0.05 A. The M–O stretching frequencies are very similar for the two structures. The H–M bond lengths and stretching frequencies in HMO are very similar to the same properties in the corresponding diatomic molecule. Analysis of the electronic structures suggests that HBeO has two polarized covalent bonds formed from the sp hybrids of Be; HMgO has one covalent bond (between H and Mg) and one ionic bond and can be viewed as (HMg)+O−, HCaO has two ionic bonds represented by H−Ca2+O−.

The energy surface of CaOH has been studied and an isomer of the form of HCaO with an electronic state of 2Σ+ symmetry has been found. It is linear with H–Ca and Ca–O bond lengths of 2.021 and 2.002 A, respectively, at the CISD level with a basis set of triple‐zeta plus double polarization quality. The overall electronic structure is largely ionic and can be described as H(1s2)Ca(4s0)O(2s22p2x2p2y2p1z) (H−Ca2+O−). The minimum on the energy surface is well defined and the harmonic vibrational frequencies have been calculated. The O–H bond length and stretching frequency and the bending frequency of CaOH have also been calculated and compared with experiment.

The thermal motion of CD+4 in the gas phase at 4 and 40 K is studied by means of a molecular dynamics method using the MP2/6‐31G(d,p) force field. The thermal motion of CD+4 in the temperature range from 4 to 40 K does not involve a deuterium permutation on the picosecond time scale. The electronic energy fluctuations of CD+4 due to thermal motion are less than 0.7 kcal/mol at 4 K and 1.6 kcal/mol at 40 K, which is less than the energy barrier (3.4 kcal/mol at MP2 level) required for the permutation between two energy minimum C2v structures. On the basis of these calculations it is suggested that the observation of four equivalent hydrogen atoms in the ESR spectrum of CH+4 can be attributed to fast quantum tunneling, with an estimated rate of 4.3×106 s−1 for CH+4 and 1.6×103 s−1 for CD+4.

The internal motion of benzene is studied in the gas phase at 30 and 300 K and in the liquid phase at 300 K by means of a molecular dynamics method based on a hybrid of quantum and molecular mechanical force fields. The fluctuations of bond lengths, bond angles and torsional angles are calculated and compared with available experimental data. A conformational analysis for the planar, boat, chair and twisted structures of benzene is carried out and liquid benzene is found to be effectively a planar hexagon at room temperature.

The hyperfine structure of the 14NH2 radical is investigated by means of multireference single and double configuration interaction (MRCI) techniques. Particular attention is paid to the dependence of the coupling constants on the basis set, reference space, and configuration selection energy threshold. It is found that convergence can be obtained only if more than 83 reference configurations are included with an energy threshold of at least 10−7 hartree. With up to 126 reference configurations, an energy threshold smaller than 10−8 hartree and an uncontracted (13s8p2d/8s2p) basis set, the MRCI isotropic couplings (27.44 and −68.47 MHz for N and H, respectively) are in very good agreement with experimental data (27.9 and −67.2 MHz, respectively).

The interactions between an alkali metal atom (Li, Na, K) and acetylene/vinylidene are investigated by means of gradient corrected (‘PWP’) density functional theory. Particular emphasis is made on the determination of equilibrium geometries, energetics and atomic hyperfine structures of the MHCCH van der Waals addition complexes, and the MHCCH and MCCH2 charge-transfer complexes. The PWP optimized geometries are found to be similar to previous ab initio data, and the generated hyperfine structures are in excellent accord with experimental matrix isolation ESR data recorded at 4 K. In the charge transfer complex formation, the metal atoms are found to donate 0.6–0.8 electron from an excited p orbital to the antibonding CC π∗ orbital of the hydrocarbon fragment. The calculated trend in charge donation matches the order of increasing atomic s → p excitation energies, K < Li < Na. This is the first application of the present method to metal atom hyperfine structures, and the results are promising for future studies.

The electronic structures of the Rydberg and related molecules belonging to the series [Formula: see text], NH4, and [Formula: see text] and H3O+, H3O, and H3O− have been investigated by analysis of the electron densities generated by the Hartree–Fock method (HF), the quadratic configuration interaction method including single and double excitations (QCISD), the coupled-cluster method including double excitations (CCD), and the density functional method with the Becke–Perdew gradient correction functionals (DFT-BP). Adding an electron to the stable cation core ([Formula: see text] and H3O+) results in an increase in the bond length, a larger increase in the distance of the bond critical point (BCP) from the nitrogen atom than from the hydrogen atom, a decrease in the electron density at the bond critical point, and an increase of the Laplacian of the electron density at the BCP. These trends are consistent with a decrease in the electronegativity of the central atom and a weakening of the bond as electrons are added to diffuse Rydberg orbitals. There is little effect, however, when a second electron is added to form the double Rydberg molecules ([Formula: see text] and H3O−). For NH4, the preferred geometry corresponds to a high atomic energy associated with the hydrogen basin, and to a low kinetic energy and a high potential energy in the basin. The high atomic energy associated with the hydrogen basin is attributed to the dominant contribution of the electron–nuclear attraction energy. Similar effects are observed in the Jahn–Teller distorted molecules: [Formula: see text] and [Formula: see text]. The DFT-BP method yields results in qualitative agreement with the results of the QCISD and CCD methods; however, it underestimates the bond strengths of the Rydberg molecules relative to high-level ab initio methods.

Density functional theory calculations of isotropic couplings of the atoms B to O are carried out with a variety of functional forms and basis sets. It is shown that the atomic isotropic coupling constants are very dependent on functional form, auxiliary basis set and orbital set. The calculations on the molecular systems NH 2 and H + 3 show less dependence for isotropic couplings and almost independence for anisotropic couplings. It is found that the combination of Perdew and Wang's non-local exchange functional and Perdew's correlation correction functional with a [7s, 6p, 2d]/(5, 2; 5, 2) orbital/auxiliary basis set gives results in semiquantitative agreement with experiment and MRSDCI calculations.

The influence of up to six Ne atoms on the isotropic hyperfine (hf) coupling constants of the hydrogen atoms in CH+4 is investigated using density functional theory. It is found that at very short NeH distances (R = 1.8 A), the hf structure is strongly affected, whereas at the more realistic van der Waals distance (R ≈ 2.5 A), the methane radical experiences a very weak field from the surrounding Ne atoms. At larger distances, the effects of the Ne matrix are negligible. It is furthermore found that at small R, the changes in aiso (H) are strongly dependent on the relative positions of the Ne atoms.

The variation of the charge and intracule densities of singly excited states of two‐electron ions with respect to spin multiplicity, nuclear charge, degree of excitation, and angular momentum quantum number is studied systematically. The n 1S, n 3S, n 1P, n 3P, n 1D, and n 3D states with n=3–6 are considered for all the ions from He through Ne8+ using highly accurate explicitly correlated wave functions. Special attention is paid to spin multiplicity differences, that is differences between the densities of a pair of states arising from the same electron configuration of the same ion.

Self‐consistent field (SCF) and complete active space multiconfiguration (CAS‐MCSCF) calculations with extended basis sets are used to show that the electron–nuclear attraction energy is the dominant contributor to the total energy lowering associated with the Jahn–Teller distortion of BH+3. Furthermore the Jahn–Teller distortion raises the interelectronic and internuclear repulsion energies. These observations are consistent with a contraction of the molecular framework and electron cloud. In the context of the relationship between the electron distribution and the Jahn–Teller distortion direction, it is observed that the electron density associated with the canonical highest occupied molecular orbital (HOMO) of the D3h constrained BH3 radical has C2v symmetry, which corresponds to the preferred point group of the Jahn–Teller distortion in BH+3. The effect of electron correlation on the electron density distribution is shown to be inconsequential for the prediction of the direction of the Jahn–Teller dis...

Multireference configuration interaction calculations for the low-lying electronic states of C2 (a 3Πu and A 1Πu) and O2 (X 3Σg−, a 1Δg) over a range of internuclear separations reveal that the singlet-triplet electron-electron repulsion energy differences tend to have the same sign as the singlet-triplet kinetic energy differences, while the corresponding electron-nuclear attraction energy differences tend to have the opposite sign at a given geometry. Adiabatic energy component differences for C2 and O2 show that the electron-electron repulsion in the triplet state is greater than in the singlet state, while there is more electron-nuclear attraction in the triplet than in the singlet state. The latter observations are consistent with the results for atomic states arising from the same configuration, but are contrary to the traditional explanation of Hund's rule.

Extensive configuration‐interaction calculations with double‐zeta plus polarization and near triple‐zeta plus polarization basis sets are used to analyze the Jahn–Teller (JT) effect in the methane radical cation. Energy component analysis shows that the Jahn–Teller effect leads to a decrease in the expectation value of the electron‐nuclear attraction energy, an increase in the expectation value of the interelectronic repulsion energy, and an increase in the internuclear repulsion energy. These observations are consistent with a contraction of the electron cloud. The dominant factor in the −0.0550 hartree Jahn–Teller distortion (Td→C2v) in CH+4 is the −0.5262 hartree change in the electron–nuclear attraction energy. The differences in all energy components are large in relation to the JT distortion. Interelectronic repulsion plays a dominant role in determining the relative energies of the possible JT distorted structures, but electron correlation effects are relatively unimportant.

This paper explores the atomic contributions to the electronic vibrationless bond dissociation enthalpy (BDE) at 0 K of the central C-C bond in straight-chain alkanes (C(n)H(2n+2)) and trans-alkenes (C(n)H(2n)) with an even number of carbon atoms, where n=2, 4, 6, 8. This is achieved using the partitioning of the total molecular energy according to the quantum theory of atoms in molecules by comparing the atomic energies in the intact molecule and its dissociation products. The study is conducted at the MP2(full)6-311++G(d,p) level of theory. It is found that the bulk of the electronic energy necessary to sever a single C-C bond is not supplied by these two carbon atoms (the alpha-carbons) but instead by the atoms directly bonded to them. Thus, the burden of the electronic part of the BDE is primarily carried by the two hydrogens attached to each of the alpha-carbons and by the beta-carbons. The effect drops off rapidly with distance along the hydrocarbon chain. The situation is more complex in the case of the double bond in alkenes, since here the burden is shared between the alpha-carbons as well as the atoms directly bonded to them, namely, again the alpha-hydrogens and the beta-carbons. These observations may lead to a better understanding of the bond dissociation process and should be taken into account when locally dense basis sets are introduced to improve the accuracy of BDE calculations.

The interaction of BeX(2) (X = H, F) with water molecules has been analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. The formation of strong beryllium bonds between water molecules and the BeX(2) derivative triggers significant electron density redistribution within the whole system, resulting in significant changes in the proton donor and proton acceptor capacity of the water molecules involved. Hence, significant cooperative and anti-cooperative effects are present, explaining why there is no case in which the global minimum corresponds to a tetracoordinated beryllium atom. In fact, the most stable clusters can be viewed as the result of the attachment of BeX(2) to the water trimer and the water dimer, respectively, and not as the result of the solvation of the BeX(2) molecule. We have also shown that the decomposition of the interaction energy into atomic components is a reliable quantitative tool to describe all the closed-shell interactions present in the clusters investigated herein, namely hydrogen bonds, beryllium bonds and dihydrogen bonds. Indeed, we have shown that the changes in the atomic energy components are correlated with the changes in the strength of these interactions, and they provide a quantitative measure of cooperative effects directly in terms of energies.

The local quantum theory is applied to the study of the momentum operator in atomic systems. Consequently, a quantum-based local momentum expression in terms of the single-electron density is determined. The limiting values of this function correctly obey two fundamental theorems: Kato's cusp condition and the Hoffmann-Ostenhof and Hoffmann-Ostenhof exponential decay. The local momentum also depicts the electron shell structure in atoms as given by its local maxima and inflection points. The integration of the electron density in a shell gives electron populations that are in agreement with the ones expected from the Periodic Table of the elements. The shell structure obtained is in agreement with the higher level of theory computations, which include the Kohn-Sham kinetic energy density. The average of the local kinetic energy associated with the local momentum is the Weizsacker kinetic energy. In conclusion, the local representation of the momentum operator provides relevant information about the electronic properties of the atom at any distance from the nucleus.

The bonding interaction between non-hydrogen-bonded (NHB) hydrogen atoms and the non-nuclear attractor (NNA) observed in cavity-bound anionic water clusters has been investigated using ab initio methods. The clusters range in size from four to ten water molecules and generally consist of several water molecules hydrogen-bonded in a ring, with two of these rings stacked such that the NHB hydrogen atoms are directed towards a central cavity. Analysis of the electron density using the theory of atoms in molecules yields information about the bonding interaction that cannot be obtained with conventional computational techniques. The results indicate that this novel interaction, Ha...NNA, is a weak, closed-shell interaction. Further analysis of the atomic properties of the NHB hydrogen atoms indicate that this novel bond cannot be classified as a hydrogen bond. On the basis of the results from the current investigation, this interaction is best described as a weak dipole-ion interaction between the positively charged NHB hydrogen atoms and the negatively charged NNA.

The K–L intershell correlation effects for both singlet and triplet states arising from the 1s2s electronic configuration of the helium atom are analyzed in detail with the aid of the complete (nonreduced) interelectronic probability density and the local Coulomb hole functions. The calculations for both singlet and triplet states suggest the existence of nuclear Coulomb well channels similar to those recently described for the intrashell case of Berry and Krause. When the reference electron is close to the nucleus, the 2 1S Coulomb hole resembles the ground state Coulomb hole, whereas this feature is lost in the 2 3S state due to the effect of exchange correlation. Electron correlation is shown to reduce the nuclear shielding provided by the K‐shell distribution and, as a consequence, to lead to a relative shrinking of the outer shell density. Other differences between the ground state and excited state correlation effects are described.

Ab initio SCF MO calculations for the hydrogen-bonded complexes between nitriles and hydrogen fluoride suggest a strong linear relationship between the charge density at the hydrogen-bond critical point and the hydrogen-bond energy. Further investigation of the topological properties of the charge density indicates that the generalization of the bond-length-bond-order relationship of CC bonds due to Bader et al. may be extended to intermolecular hydrogen bonding. Calculations at the 6–31G ** level, including complete geometry optimization, are reported for the complexes, where R  H, Li, F, Cl, HO, LiO, NC, HCC, CH 3 and CH 3 O.

The radius of the Coulomb hole in an atom or molecule is discussed in general terms and illustrated in detail for the ground state of the helium isoelectronic sequence. The calculations demonstrate a general tendency for the radius of the Coulomb hole to increase as a function of the distance of the reference electron from the nucleus and the existence of a nonzero lower limit (0.800 a.u. in He and 0.111 a.u. in Mg10+) for the radius of the Coulomb hole. This limit is slightly larger than the radial distance at which the radial density distribution reaches its maximum. By expressing the correlation energy of an atom in terms of certain one and two electron expectation values, the relationship between the correlation energy and the size and shape of the Coulomb hole is discussed. Detailed comparisons for the ground and 23S states of He are used to demonstrate the relationship.

An electrostatic model, according to which the electronegativity of an atom is identified with the electrostatic force between the effective nuclear charge and an electron at a distance equal to the relative radius of the atom, is used to calculate a nonempirical electronegativity scale for the first 54 elements. The relative radius is calculated from the free‐atom wave function by use of the density contour approach, while intergration of the radial density from the nucleus to the relative radius yields the effective nuclear charge. In contrast to the empirical methods, the nonempirical electrostatic method is not only applicable to all elements, but treats all elements equally. In addition to correlating well with empirical scales, the proposed model is consistent with the major conclusions of the density functional approach of Parr and his co‐workers. In particular, the model is easily extended to yield valence state electronegativities.

Analytical expansions for the radial density D(r), as calculated within the Hartree–Fock approximation, are reported for the neutral atoms from helium to xenon. As part of a brief discussion of one-electron properties which may be calculated from D(r) or the related density distribution ρ(r), expectation values for rn, n = −2, −1, 1, 2, 3, 4, and 5 are presented.

The density difference ΔD(r1), defined as the difference between the exact and the Hartree-Fock radial density functions is reported for the 2 3S and the 2 1S excited states of the helium atom. The density differences in both cases resemble the inverse of the helium ground-state density difference, but are more long-ranged due to the diffuseness of the excited states. The effect of electron correlation on one-electron distributions is also demonstrated by comparison of (r1n) expectation values and several indices of the relative sizes of atoms. Electron correlation leads to a contraction of the Hartree-Fock electron clouds.

The topological properties of the charge density of the hydrogen-bonded complexes between nitrites and hydrogen chloride correlate linearly with theoretical estimates of the hydrogen-bond energy. At the 6-31G** level, the hydrogenbond energies range from a low of 10 kJ mol m NCCN—HC1 to a high of 38 kJ mol in LiCN—HCl. A linear relationship between the charge density at the hydrogen-bond critical point and the NH internuclear distance of the RCN—HC1 complexes indicates that the generalization of the bond-length-bond-order relationship of CC bonds due to Bader, Tang, Tal and Biegler-Konig can be extended to intermolecular hydrogen bonding.

A study of the 1 B 1 and 3 B 1 states of CH2 has been carried out, with the aim of evaluating the singlet–triplet differences in the total energy and its components. These were examined in connection with the interpretation of Hund’s multiplicity rule. Although vertical results near equilibrium tend to vindicate the new explanation of Hund’s rule, the effect of the molecular virial theorem must be considered at large separations. Also, for adiabatic comparisons, the effect of bond lengths on the energy components must be considered. Correlation energy was found to have a significant effect on vertical energy component differences at certain geometries.

Ab initio SCF MO calculations are reported for benzene, s -triazine, borazine and boroxine. The Laplacian of the charge density and the Mulliken population analysis procedure demonstrate that the delocalization of the π electrons decreases and the polarity of the ring bonds increases substantially as the atoms in the ring become more dissimilar. Several other properties, including distortion of the ring angles, puckering of the ring and nuclear quadrupolar coupling constants, emphasize the different chemical properties within the isoelectronic series.

Potential curves for the ground and all excited singlet states which dissociate into the ground states of the atoms, Cl(2pu) and F(2Pu), or into the ionic species, Cl+(1Dg) and F−(1Sg), have been calculated by means of the multi-reference configu interaction (MRD CI) method. The Gaussian basis set consists of a total of 70 contracted functions, including Rydberg and bond functions. The calculations show that adequate treatment of Rydberg-valence mixing is essential for an understanding of the ClF electronic absorption spectrum. The calculated Do value for the ground state is 2.47 eV compared to the experimental value of 2.617 eV. Transitions of potential spectroscopic interest include the dipole allowed 1Σ+ → 1/gP transition, having a vertical transitio energy of 8.92 eV, and the allowed 1Σ+ → 1Σ+ transition, with a vertical transition energy of 10.3 eV.

The effect of electron correlation on the Politzer–Parr procedure for partitioning the electron density of an atom into core and valence regions is investigated by use of configuration interaction (CI) wave functions. Although the effect is generally small, substantial qualitative differences are observed between the 3, 4, and 6 electron cases. Analytical Hartree–Fock (HF) wave functions are used to reveal the trends in isoelectronic and isonuclear sequences. Correlated wave functions, which explicitly include the interelectronic distance, are used to study the effect of spin and electronic excitation on the populations and dimensions of the core and valence regions in the 1sns excited states of He.