Ab Initio SCF Calculations Research Articles

Water Molecules in Carbon 70, (H<sub>2</sub>O)<sub>3</sub>@C<sub>70</sub>

Endohedral fullerene studies are the fascinating one, particularly with Carbon 60 and Carbon 70. Water molecules inside fullerenes alter their cage structure, reorientations make them to play a lot in charge distribution. In this line we are presenting our work on Carbon 70 with three water molecules inside. Ab initio SCF calculations are carried out for the fullerene Carbon 70 and Carbon 70 with three water molecules. Carbon 70 is a rugby ball structure, when three water molecules are added inside it, dissociation of charges takes place. Unusual flip flop circular hydrogen bond formation takes place inside Carbon 70. The dipole moment of endohedral C70 with three water molecules has been found to be 0.53 Debye, 0.49 Debye and 0.71 Debye respectively for STO-3G, 3-21G and 6-31G basis sets. Total energies for this molecule are reported in addition to the Hydrogen bond length and bond angles of the three water molecules trapped inside C70.

Exploring NLO response of 9,10- donor-acceptor substituted Bichromophoric Anthracene Derivatives

Ab-initio finite field SCF calculations with split valence double zeta basis set indicate that 9, 10- donor-acceptor substituted anthracene derivatives have large quadratic hyperpolarizabilities for suitable combinations of donor and acceptor moieties. A difference in the first hyperpolarizabilitie s (βtotal) of isomeric organic chromophores containing the same donor and acceptor groups indicate the possible role of intramolecular charge transfer (ICT) in shaping the NLO response in these π-conjugated molecular chromophores. A correlation is sought to be established among calculated βtotal on one hand and the donor strength, the strength of the acceptor, donor-acceptor interaction and the donor-acceptor separation, on the other by using Genetic Algorithm (GA) to search through the relevant parameter space. It appears that βtotal of a molecule is dominantly determined by additive contributions from the donors and the acceptors. The results might be helpful in designing new NLO materials using the bichromophoric anthracene derivatives.

Crystal structure studies using ab-initio potential functions from partitioned ab-initio MODPOT/VRDDO SCF energy calculations. I. N2 and CO2 test cases. II. Nitromethane, CH3NO2

Several years ago we embarked on a project to calculate optimal crystal-packing and crystal-structure parameters based on potential functions from energy-partitioned ab-initio intermolecular SCF calculations plus calculations of dispersion energy contributions. Our approach, aimed at evaluating the intermolecular interactions, such as in polymer propagation and in molecular crystals, and at analyzing the optimal crystal packing, is based on nonempirical ab-initio calculations for smaller molecular aggregates (monomers, dimers, trimers, etc.), partitioning the total SCF interaction energies into the different components, and then fitting these components individually to functional forms or when necessary recalculating or estimating explicitly for certain interaction components for each different unit cell dimension change. We derived and implemented a complete program, including higher order terms and special features necessary only for calculations on crystals. We also derived and implemented a complete CRYSTAL program which, given the crystal symmetry, allows us to vary and optimize the crystal-structure parameters. We carried out test crystal optimization calculations on N2 and on CO2 both to check out the entire CRYSTAL program package and to explore the extensive programs written for various ab-initio based methods for calculating reliable intermolecular interactions. We investigated such aspects as the comparison of the various intermolecular interaction terms calculated from the potential functions compared with those calculated by ab-initio wave functions. We also investigated for the CRYSTAL program how many surrounding unit cells had to be included for energy convergence. We investigated the crystal CH3NO2. We calculated the intermolecular interactions at more than 50 geometries, partitioned the interaction energies, and fit these to functional forms. The agreement of our calculated unit cell dimensions with experiment was excellent, within 1 to 2.8%. Furthermore, our method fits the partitioned ΔESCF results to atom-class-atom-class potential functions, which enables a general library of such potential functions to be built up for the molecular classes of interest.

Correlation and basis set superposition effects on three body ion-water-water interactions

Ab initio SCF and second order Moller-Plesset electronic calculations using a point charge (PC) model as well as standard basis sets for the ions (Li+, Mg++) have been performed for several different water-water geometries and distances. The PC model yields acceptable ion-water potential curves. The ab initio curves calculated with and without Basis Set Superposition Error and electron correlation corrections as well as via a classical model are compared for various water dimer orientations. Certain geometries (particularly those appropriate to the interaction of hydrated ions) gave large Basis Set Superposition Errors and large correlation effects (˜20%) even with the large basis set (Clementi – (11,7,1/6,1) [4,3,1/2,1]) used. For attractive potentials this lead to some cancellation but it is compounded for repulsive curves. The classical model used was able to predict qualitative trends in most cases indicating the nonadditivity arises primarily from an inductive effect; however, geometries with strong hydrogen-hydrogen repulsions show an attractive three body potential at short distances which cannot be understood in terms of this classic model.

The role of hydrogen bonding in some diol epoxides

The role of intramolecular hydrogen bonding in determining the properties of two diol epoxides of benzene has been investigated by means of ab-initio SCF calculations (Gaussian 70, STO-5G). These two systems were used as models for the syn and anti 7,8-diol-9,10-epoxides of benzo[a]pyrene, one or both of which are believed to be ultimate carcinogenic forms of benzo[a]pyrene. It was found that there is intramolecular hydrogen bonding in the syn diol epoxide of benzene, but not the anti isomer. This hydrogen bonding was shown to have the effect of facilitating the opening of the epoxide ring at either carbon, but particularly at C(1). These findings are fully consistent with the observed properties of the previously mentioned diol epoxides of benzo[a] pyrene and help to explain these properties.

Calculation of molecular ionization potentials: Comparison of MSXα and ESCA

Preliminary results of calculations by means of the ms Xα (multiple-scattering Xα) method on four small molecules (N2, CO, NO, CF4) are presented. Comparisons with the molecular ionization potentials measured by the esca (electron spectroscopy for chemical analysis) experiments show in general better agreement than the semiempirical calculations and about the same agreement as the ab initio scf calculations.

Ab-initio SCF calculations on pyrrole and pyrazole

Full ab-initio SCF calculations have been carried out on pyrrole and pyrazole with very large Gaussian basis sets. A preliminary analysis of the molecular wave functions has been made by looking at the total and orbital energies and some of the properties. A comparison is made between these results and previous SCF calculations performed with much smaller basis sets. The ionization energies obtained by Koopman's theorem are compared with the experimental photoelectron spectroscopic values. Very good agreement is obtained for the two highest orbitals (both of π type). The comparison of pyrrole and pyrazole with some recent SCF results for furan is also included.

Comparison for pyrrole and pyrazole of orbital energies and population analyses from ab-initio SCF, CNDO/2, INDO, extended hückel, and ARCANA calculations

The results of our recent ab-initio SCF calculations on pyrrole and pyrazole using large Gaussian basis sets are compared to those using less rigorous methods: CNDO/2, INDO, PCILO extended Huckel, and ARCANA (a type of iterative extended Huckel calculation). The ab-initio orbital energies are higher in energy than all those resulting from less rigorous methods. For the two highest occupied orbitals, both of π type, (pyrrole— 1a2 and 2b1; pyrazole—3a″ and 2a″) there was excellent agreement between the measured photoelectron spectroscopic ionization potentials and the Koopman's theorem values from the ab-initio results. Thus the orbital energies calculated by the less rigorous methods all lie too low in energy. The CNDO/2 and INDO orbital energies also span too large a width for the valence band compared to the ab-initio results. The gross atomic populations from the CNDO/2, INDO and PCILO methods show discrepancies from the ab-initio results for both the heteroatoms and the hydrogens. Total overlap populations calculated from the ab-initio SCF wave functions show large negative overlap populations between nonbonded ring atoms. Using the off-diagonal density matrix elements from CNDO/2 and INDO wave functions as if they were true off-diagonal elements from non-ZDO calculations, scaling them (shown previously to lead to reasonable values for TOPS between bonded atoms) led to TOPS between nonbonded ring atoms that were essentially zero. Wiberg bond indices, by definition only capable of giving positive numbers, also cannot reproduce these negative overlap populations. The extended Huckel and ARCANA methods, which are non-ZDO methods, both gave negative tops between nonbonded ring atoms, although not as large as those which resulted from ab-initio calculations.

Proposed use of charge-density contour maps from Ab initio SCF calculations to calibrate sphere sizes for the MS-Xα method

Proposed use of charge-density contour maps from Ab initio SCF calculations to calibrate sphere sizes for the MS-Xα method

Quantum-mechanical studies of environmental effects on biomolecules. III. Ab initio model studies of the hydration of peptides and proteins

Ab initio SCF calculations on the hydration of the peptide linkage and of the hydrophilic amino acid side chains of proteins are used to evaluate the binding energies and the most likely positions of the water molecules of the first hydration shell. The results are compared with the very scarce available experimental data.

Intermolecular interactions: Reproduction of the results of ab initio supermolecule computations by an additive procedure

An additive systematics for the rapid computation of intermolecular interactions has been worked out. The interaction energy is computed as a sum of four components. The electrostatic component is obtained by using the overlap multipole expansion of the SCF density distribution of the isolated ligands, and the other components (polarization, dispersionlike, and repulsion) have been calibrated in order to reproduce the results of ab initio SCF computations on the systems Na+—H2O, K+—H2O, H2O—H2O, and NH4+—H2O. A series of cation-ligand interactions is then investigated with this systematics, and the results, compared with the corresponding ab initio SCF ones, are seen to reproduce satisfactorily both the values of the interaction energies and the essential features of the binding.

An Approach to Hydration of Model Silica Materials by Exploring Their Multiple Minima Hypersurfaces. The Role of Entropy of Association

The influence of molecular water on the structure and formation of silica clusters is modeled with the use of the MMH (multiple minima hypersurfaces) approach. It combines quantum chemical Hamiltonians for the calculation of the internal energy with statistical modeling and formulas for the calculation of thermodynamic functions of association. The structures of the most probable clusters of hydration and some properties of the association with water are proposed. Different simple structures are calculated to consider the entropy of association in place of the simpler approach of a single "global minimum". Ab initio, DFT, and semiempirical calculations of the structures and relevant reactions of silica fragments are also reported, confirming the reliability of the results on very different grounded quantum mechanical methods. Particularly, it has been shown that semiempirical PM3 Hamiltonian is reliable for silica cluster calculations of this kind, in comparison with accurate ab initio SCF and other DFT calculations. Apparently, the well-known systematic failures of this Hamiltonian are absent in this kind of structures and interactions. The increasing hydrophobic character of neutral silica clusters appears as a result of the free energies of association, and therefore, it originates on entropy. It is remarkable that the simple global minimum approach currently used in works of molecular modeling on this kind of compounds, where several hydrogen bridges have a place to exist with several different conformations, must be taken with caution given the remarkable entropy of association of such systems.

Spectroscopic and ab initio study of an intramolecular charge transfer (ICT) rhodanine derivative

The steady state absorption and emission studies of the rhodanine derivative 5-( p-dimethylaminobenzylidene) rhodanine (PDR) have been investigated. A dual fluorescence associated with an intramolecular charge transfer (ICT) phenomenon is reported. The characteristic emission band at low temperature (77 K) has also been reported. In support of the experimental results ab initio SCF and DFT calculations have been performed. The theoretical calculations also indicate that the chosen compound has significant non-linear optical (NLO) properties.

Towards understanding the molecular internal rotations and vibrations and chemical reactions through the profiles of reactivity and selectivity indices: an ab initio SCF and DFT study

Ab initio SCF and DFT(B3LYP) calculations are performed with 6–311G** basis sets for obtaining insights into molecular internal rotations in HXNX (X = O,S), different vibrational modes in water and double proton transfer reaction in (HONO)2. While chemical reactivity is analyzed in terms of the profile of the global reactivity parameters, such as energy, chemical potential, hardness, polarizability, molecular valency and electrophilicity indices, the site selectivity is understood through the variations in local descriptors, such as the Fukui function and atomic valency. Principles of maximum hardness and molecular valency and the minimum polarizability principle are found to be valid in almost all cases. Rotational isomerization reactions can be better characterized by making use of the maximum hardness principle along with Hammond's postulate. Extremum points in electrophilicity during internal rotations, vibrations and chemical reaction can be located from those of chemical potential and hardness. The Fukui function and atomic valency show inverse behaviour in most cases.

Heterocomplexes in liquid mixtures MeOH—HF

The IR spectra and densities of MeOH—HF liquid mixtures were measured for component molar ratios ranging from 12 : 1 to 1 : 3. The IR spectra of all of the solutions exhibit bands with maxima at 3500, 2600, and 1800 cm–1 and continuous absorption (CA) in the region of 3500—1300 cm–1. The intensities of these bands and the CA increase with an increase in the HF concentration. A similar behavior was found for the difference between the experimental solution density and the additive sum of the solution component densities. Ab initio SCF calculations of the (MeOH)5–n (HF) n cyclic complexes (n = 0—5) were performed using the 6-31G basis set. The observed properties of MeOH—HF liquid mixtures may be due to the formation of heterocomplexes with a stable cyclic fragment consisting of more than four MeOH and HF molecules.

A Matrix Isolation and ab Initio Study of the Hydrogen Peroxide Dimer

The infrared spectrum of the hydrogen peroxide dimer has been studied in argon matrices at 17 K. The spectra have been assigned from concentration dependencies, experiments with different H/D ratios, photolysis experiments, and quantum chemical ab initio SCF calculations. The dimer has a cyclic structure with both components acting as hydrogen bond donors and as hydrogen bond acceptors. There is a significant coupling between the two bonded OH stretches in the two molecules and a relatively strong coupling between the corresponding HOO bends.

C32: Computations of Low-Energy Cages with Four-Membered Rings

C32 cages built from four-, five-, six-, and seven-membered rings are computed. The computations are primarily performed with semiempirical quantum-chemical methods (AM1, PM3, SAM1), and altogether 199 cages are optimized. The energetics is further checked through ab initio HF SCF computations with the standard 3-21G basis set, and also by density functional theory at the B3LYP level in the standard 6-31G* basis set. All five levels of theory suggest a D4d cage (two four-membered rings, eight pentagons, eight hexagons) as the lowest-energy structure. Temperature effects are treated in the terms of partition functions so that the entropy contributions are considered accordingly. The thermodynamic treatment points out five cages significantly populated at high temperatures. At very high temperatures the structure lowest in energy is not the most abundant isomer. There are just six conventional fullerenes C32, built exclusively from pentagons and hexagons, however, only two of them show significant populations at high temperatures. The remaining three relatively stable cages contain at least one four-membered ring. No structure with a heptagon shows a non-negligible concentration at high temperatures. The study suggests that in the non-IPR region the quasi-fullerene cages with four-membered rings can in some cases be more important than the conventional fullerenes built from pentagons and hexagons only.

Conformational analysis of cis-Δ1, -Δ2, -Δ3 and -Δ4-tetrahydrophthalic anhydrides by semiempirical and ab initio molecular orbital calculations

Semiempirical and ab initio molecular orbital calculations have been used to study the conformational geometries and the relative stability of the conformers of cis-Δ1, -Δ2, -Δ3 and -Δ4-tetrahydrophthalic anhydrides (THPA). The total energy and the structure of the transition states for the interconversion of conformers were calculated with PM3 semiempirical method. It is found that PM3, AM1 and MNDO semiempirical schemes underestimate the stability of cis-Δ1-THPA, contrary to experimental findings. The ab initio SCF calculations performed at the Restricted Hartree–Fock (RHF) and MP2 levels using effective core pseudo-potential functions with basis sets contracted to 31 split valence (PS-31G) and (PS-31G∗), are in good agreement with the available experimental evidence, e.g. the cis-Δ1-THPA is, thermodynamically, the most stable of all the cis-isomeric anhydrides. The ab initio results indicate that the energy between the conformers changes as a function of the AO basis set. The ground states conformations of cis-Δ1-THPA are increasingly preferred in the more extended basis sets including d-polarized functions on carbon and oxygen atoms. For cis-Δ4-THPA, the folded conformation is calculated to be favored over the open conformation in agreement with the results of NMR analysis of cis-Δ4-tetrahydrophthalic anhydride. The preference of this conformation is also consistent with the crystal structure recently determined by X-ray diffraction technique. The structure analysis of the different conformations at different levels of ab initio calculation, as well as with semiempirical methods, shows that the cyclohexene ring adopts a boat form in cis-Δ4-THPA, a half-chair form in cis-Δ1-THPA and deformed sofa forms in both cis-Δ3-THPA and cis-Δ2-THPA; the anhydride ring being flattened in cis-Δ1-THPA and cis-Δ4-THPA and more twisted in cis-Δ3-THPA and cis-Δ2-THPA. Comparison is made with structures of related compounds.

The hydrogen-bonded complex H3P···HCl investigated by a combination of rotational spectroscopy and ab initio SCF calculations

Ground-state rotational spectra were observed for the four symmetric-top isotopomers H3P···H35Cl, H3P···H37Cl, H3P···D35Cl and D3P···H35Cl and the four asymmetric-rotor isotopomers H2DP···H35Cl, H2DP···H37Cl, HD2P···D35Cl and H2DP···D35Cl of a complex formed in mixtures of phosphine and hydrogen chloride. Rotational constants, centrifugal distortion constants, Cl nuclear quadrupole coupling constants and spin–rotation coupling constants were determined through spectral analyses. Interpretation of various of the spectroscopic constants with the aid of models of the complex provided information about its symmetry and geometry, about the strength of the intermolecular binding and about the subunit dynamics. The complex has C3v symmetry and the order of the nuclei is unambiguously established as H3P···HCl. Ab initio calculations carried out with three different basis sets [6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ] and with electron correlation taken into account at the MP2 level were used to optimise the geometry under the assumption of C3v symmetry and indicated that only small geometrical distortions of the PH3 and HCl subunits accompany formation of the complex. By assuming that similar distortions are also appropriate to experimental geometries of the subunits, the rotational constants of the symmetric-top isotopomers of the complex were used to establish the distances r(P···H) and r(P···Cl). Although r(P···H) was constant at 2.5802(2) A for the three species based on PH3, this distance shortened to 2.5736(5) A in D3P···H35Cl. An alternative analysis indicated that r(P···Cl) shortened by 0.0014 A per D atom substituted for phosphine H atoms in H3P···H35Cl, an effect attributed to an increased magnitude of the electric dipole moment of PH3 as it is deuteriated. The centrifugal distortion constants DJ and D = 2DJ+DJK were used to determine values of the quadratic intermolecular stretching and bending force constants kσ and kθθ, respectively. The latter also yielded an average value 〈θ2〉1/2 of the PH3 oscillation angle θ, which decreased significantly for D3P···H35Cl relative to the (constant) value for species based on PH3. After correction of the Cl nuclear quadrupole coupling constant of H3P···H35Cl for the effects of the electric field gradient due to PH3 by using estimates from the MP2/aug-cc-pVTZ calculation, the corrected value was used to give the average βav = cos−1〈cos2β〉1/2 = 15° for the oscillation angle of the HCl subunit.

Interaction of neutral and zwitterionic glycine with Zn2+ in gas phase:ab initio and SIBFA molecular mechanics calculations

The interaction of Zn2+ with glycine (Gly) in the gas phase is studied by a combination of ab initio and molecular mechanics techniques. The structures and energetics of the various isomers of the Gly–Zn2+ complex are first established via high-level ab initio calculations. Two low-energy isomers are characterized: one in which the metal ion interacts with the carboxylate end of zwitterionic glycine, and another in which it chelates the amino nitrogen and the carbonyl oygen of neutral glycine. These calculations lead to the first accurate value of the gas-phase affinity of glycine for Zn2+. Ab initio calculations were also used to evaluate the performance of various implementations of the SIBFA force field. To assess the extent of transferability of the distributed multipoles and polarizabilities used in the SIBFA computations, two approaches are followed. In the first, approach (a), these quantities are extracted from the ab initio Hartree–Fock wave functions of glycine or its zwitterion in its entirety, and for each individual Zn2+-binding conformation. In the second, approach (b), they are assembled from the appropriate constitutive fragments, namely methylamine and formic acid for neutral glycine, and protonated methylamine and formate for the zwitterion; they undergo the appropriate vector or matrix rotation to be assembled in the conformation studied. The values of the Zn2+–glycine interaction energies are compared to those resulting from ab initio SCF and MP2 computations using both the all-electron 6-311+G(2d,2p) basis set and an effective core potential together with the valence CEP 4-31G(2d) basis set. Approach (a) values closely reproduce the ab initio ones, both in terms of the total interaction energies and of the individual components. Approach (b) can provide a similar match to ab initio interaction energies as does approach (a), provided that the two constitutive Gly building blocks are considered as separate entities having mutual interactions that are computed simultaneously with those occurring with Zn2+. Thus, the supermolecule is treated as a three-body rather than a two-body system. These results indicate that the current implementation of the SIBFA force field should be adequate to undertake accurate studies on zinc metallopeptides. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 963–973, 2000