Comparison of DFT methods for molecular structure and vibration spectra of ofloxacin calculations

Comparison of the performance of different density functional theory (DFT) methods at various basis sets in predicting molecular and vibration spectra of ofloxacin was reported. The methods employed in this study comprise six functionals, namely, mPW1PW91, HCTH, LSDA, PBEPBE, B3PW91 and B3LYP. Different basis sets including LANL2DZ, SDD, LANL2MB, 6-31g, 6-311g and 3-21g were also examined. Comparison between the calculated and experimental data indicates that the mPW1PW91/6-311g level afford the best quality to predict the structure of ofloxacin. The results also indicate that B3LYP/LANL2DZ level show better performance in the vibration spectra prediction of ofloxacin than other DFT methods.     

Ab initio molecular orbital calculations

A series of non-empirical calculations on furan, pyrrole and 1,2,5-oxadiazole are reported in which the effect of polarisation functions added to the minimal 7s 3p basis on each atom is studied. The effect on these planar molecules is largely through the rather than the-system. A comparison with the results of work with scaled functions is reported. Both series are shown to lead to much improved agreement with the electron spectroscopy energy levels. The effect on the dipole moments of these changes in basis is more variable but, with the exception of furan, the agreement with experiment is improved in the present method.

---

Zusammenfassung Für die Moleküle Furan, Pyrrol und 1,2,5-Oxadiazol wurde eine Reihe von nichtempirischen Rechnungen durchgeführt, in denen der Einfluß von zusätzlichen Polarisationsfunktionen zur minimalen 7s 3p-Basis an jedem Atom untersucht wird. Die Ergebnisse werden mehr durch die Art der Beschreibung des Systems der-Elektronen als durch diejenige der-Elektronen beeinflußt. Ein Vergleich mit den Ergebnissen bei Verwendung skalierter Funktionen wird durchgeführt. Beide Reihen von Ergebnissen zeigen eine verbesserte Übereinstimmung zu den Energiemeßwerten der Elektronenspektroskopie. Die Änderungen des berechneten Dipolmoments bei derartigen Basisvariationen sind größer als bei früheren Methoden. Die Übereinstimmung mit dem Experiment wird, mit Ausnahme von Furan, jedoch verbessert.

     

Ab Initio molecular orbital calculations

Linear combinations of Gaussian orbitals were contracted to minimal basis sets. The binding energies (B.E.) obtained for the three compounds furan, pyrrole and 1,2,5-oxadiazole are 0.75, 0.84 and 0.26 a.u. respectively. Correlation of these figures and those obtained in related calculations leads to the relationship (B.E.)_exp.=0.7347 (B.E.)_calc.+633 (Kcal/mole). The photoelectron spectra of these compounds are extended and correlated with the orbital energies; the -electron excitations are given by (I.P.)_exp.=1.00 (I.P.)_calc.–2.20 eV. Mulliken population analyses give dipole moments in reasonable agreement with experiment.

---

Zusammenfassung Linearkombinationen von Gauß-Orbitalen wurden zu minimalen Basissätzen kontrahiert.Die Bindungsenergien (B.E.) für die drei Komponenten Furan, Pyrrol und 1,2,5-Oxadiazol haben die Werte: 0,75, 0,84 und 0,26 a.u. Setzt man diese Werte zu den berechneten in Beziehung, so erhält man die Gleichung: (B.E.)_exp.=0,7347 (B.E.)_calc.+633 (Kcal/Mol).Die Photoelektronenspektren dieser drei Komponenten werden erweitert und mit den Orbitalenergien korreliert; die -Elektronen-Anregungsenergien werden durch die Beziehung: (I.P.)_exp. =1.00 (I.P.)_calc.–2,20 eV gegeben. Mulliken-Populations-Analysen ergeben die Dipolmomente in bemerkenswerter Übereinstimmung mit experimentellen Werten.

     

Semi-empirical molecular orbital calculations for pyridinealdoximes and acetamidopyridines

Tautomerism of pyridinealdoximes and acetamidopyridines is discussed with regard to their electronic absorption spectra. The spectra of both neutral and ionic forms of the molecules are interpreted by means of the Pariser—Parr—Pople type of calculation in a satisfactory way.The stability of 2- and 4-pyridinealdoxime tautomers is investigated using the CNDO/2 method. The lactim are more stable than the lactam forms of the molecules. The molecules having the substituent group of the syn-configuration are more stable than the molecules with other configurations of the substituent.     

Comparison of SCC-DFTB and NDDO-based semiempirical molecular orbital methods for organic molecules

Extensive testing of the SCC-DFTB method has been performed, permitting direct comparison to data available for NDDO-based semiempirical methods. For 34 diverse isomerizations of neutral molecules containing the elements C, H, N, and O, the mean absolute errors (MAE) for the enthalpy changes are 2.7, 3.2, 5.0, 5.1, and 7.2 kcal/mol from PDDG/PM3, B3LYP/6-31G(d), PM3, SCC-DFTB, and AM1, respectively. A more comprehensive test was then performed by computing heats of formation for 622 neutral, closed-shell H, C, N, and O-containing molecules; the MAE of 5.8 kcal/mol for SCC-DFTB is intermediate between AM1 (6.8 kcal/mol) and PM3 (4.4 kcal/mol) and significantly higher than for PDDG/PM3 (3.2 kcal/mol). Similarly, SCC-DFTB is found to be less accurate for heats of formation of ions and radicals; however, it is more accurate for conformational energetics and intermolecular interaction energies, though none of the methods perform well for hydrogen bonds with strengths under ca. 7 kcal/mol. SCC-DFTB and the NDDO methods all reproduce MP2/cc-pVTZ molecular geometries with average errors for bond lengths, bond angles, and dihedral angles of only ca. 0.01 A, 1.5 degrees , and 3 degrees . Testing was also carried out for sulfur containing molecules; SCC-DFTB currently yields much less accurate heats of formation in this case than the NDDO-based methods due to the over-stabilization of molecules containing an SO bond.     

An efficient molecular orbital approach for self-consistent calculations of molecular junctions

To model electron transport through a molecular junction, we propose an efficient method using an ab initio self-consistent nonequilibrium Green's function theory combined with density functional theory. We have adopted a model close to the extended molecule approach, due to its flexibility, but have improved on the problems relating to molecule-surface couplings and the long-range potential via a systematic procedure for the same ab initio level as that of Green's function. The resulting algorithm involves three main steps: (i) construction of the embedding potential; (ii) perturbation expansion of Green's function in the molecular orbital basis; and (iii) truncation of the molecular orbital space by separating it into inactive, active, and virtual spaces. The above procedures directly reduce the matrix size of Green's function for the self-consistent calculation step, and thus, the algorithm is suitable for application to large molecular systems.     

Semiempirical molecular orbital calculations on boranes

Traditional Pariser-Parr-Pople and variable electronegativity calculations have been carried out on C₆H₅B(OR)₂ and p-CH₃OC₆H₄B(OR)₂, and the results compared with calculations for C₆H₅BR₂. It is concluded that the VE-SCF method offers a real advantage over the simple PPP method for predicting percent charge transfer and transition intensity in cases where excited states possess substantial C.T. character. The restriction that empirically chosen parameters fit the observed transition energies and intensities of both triarylboranes and ArB(OR)₂ requires the choice of a boron VSIP greater than 2.0 eV in the fixed parameter procedure of the usual PPP-SCF-CI method for these molecules. Observed transitions in C₆H₅B(OR)₂ correlate with ¹ L _b, ¹ L _a, ¹ B _b, whereas the first absorption maximum of (C₆H₅)₃B is assigned to C.T. (¹ A ₁¹ A ₁) local C _2v symmetry.

---

Zusammenfassung PPP- und VE SCF-Rechnungen wurden für C₆H₅B(OR)₂ und p-CH₃OC₆H₄(OR)₂ durch- geführt, und die Ergebnisse wurden mit denjenigen für C₆H₅BR₂ verglichen. Es kann der Schluß gezogen werden, daß die VE SCF-Methode einen Vorteil gegenüber der einfachen PPP-Methode bietet, um den prozentualen Charge Transfer und Übergangsintensitäten in Fällen zu bestimmen, in denen die angeregten Zustände einen wesentlichen C.T.-Charakter besitzen. Die Bedingung, daß die empirisch gewählten Parameter den beobachteten Übergangsenergien und -intensitäten von sowohl Triarylboranen als auch ArB(OR)₂ angepaßt sein sollen, erfordert die Wahl eines Bor-VSIP größer als 2,0 eV im Rahmen der üblichen Parametrisierung der PPP-SCF-CI-Methode. Beobachtete Über-gänge in C₆H₅B(OR)₂ korrelieren mit ¹ L _b, ¹ L _a, ¹ B _b, wogegen das erste Absorptionsmaximum des (C₆H₅)₃B einem C.T.-Übergang (¹ A ₁¹ A ₁) lokaler C_2v -Symmetrie zugeordnet wird.

---

Résumé Des calculs traditionnels Pariser-Parr-Pople et des calculs d'électronégativité variable ont été effectués sur C₆H₅B(OR)₂ et p-CH₃OC₆H₄B(OR)₂ avec comparison aux résultats obtenus pour C₆H₅BR₂. La conclusion est que la méthode VE-SCF offre un réel avantage sur la méthode PPP simple en ce qui concerne la prédiction du transfert de charge et de l'intensité de transition pour les états excités possédant un net caractère de transfert de charge. La restriction selon laquelle les paramètres empiriques doivent permettre de reproduire les énergies de transition et les intensités des deux triarylboranes et de ArB(OR)₂, nécessite le choix d'un potential d'ionisation de l'état de valence du bore supérieur de 2 eV à celui employé dans les méthodes ordinaires. Les transitions observées dans C₆H₅B(OR)₂ sont reliées à ¹ L _b, ¹ L _a, ¹ B _b, tandis que la première absorption de (C₆H₅)₃B est attribuée à un transfert de charge (¹ A ₁¹ A ₁) de symétrie locale C_2v.

     

Flavin enzymes: molecular orbital calculations

Molecular orbital calculations were obtained for one hundred and forty-four versions of the flavin group contained in oxidase enzymes. Ten different structures were considered and the results indicate that the flavin group changes its identity from a pi electron acceptor to a pi electron donor after reduction occurs, regardless of the inductive effect of the attached protein. Furthermore, the calculations suggest both the site for reduction and the mechanism by which it proceeds.     

Semi-empirical LCAO-SCF molecular orbital calculations for selected homo-substituted benzenes

Methods of parameter compilation, for calculating the excited singlet energy levels of substituted benzenes using the Pariser-Parr-Pople method, are compared for selected cases. A process of optimisation is employed to achieve the best fit to the observed spectra of the mono-substituted benzenes. The optimum parameters are then used to calculate the spectra of the di- and tri-homo-substituted cases. Conclusions as to a best set of data are drawn from comparison of the calculated results with the observed spectra of the di- and tri-substituted cases.

---

Zusammenfassung Methoden der Parameterwahl im Rahmen des PPP-Verfahrens werden für den Fall einiger substituierter Benzole verglichen. Die Parameter werden bei den monosubstituierten Verbindungen bezüglich der experimentellen Spektren optimalisiert und zur Berechnung der entsprechenden Daten für die di- und trisubstituierten Analoga verwendet. Aus dem Vergleich dieser Ergebnisse mit dem Experiment lassen sich Rückschlüsse auf die verschiedenen Parameter-Verfahren ziehen.

---

Résumé On compare sur des cas choisis les méthodes de détermination des paramètres, pour le calcul des états excités singulets des benzènes substitués, par la méthode de Pariser-Parr-Pople. Un procédé d'optimisation permet d'obtenir le meilleur accord avec les spectres observés des benzènes monosubstitués. Ces paramètres optimum sont alors utilisés pour calculer les spectres des dérivés di et tri homosubstitués. Des conclusions sur un ensemble de données optimum sont tirées de la comparaison des calculs avec les spectres observés des composés di et tri substitués.

     

Multiple scattering mass operator method for molecular orbital calculations

An exchange-correlation potential model to be used in connection with the multiple scattering method is presented. Retaining the main advantages of the multiple scattering method with the Xα potential, particularly its low computational requirements, this new formalism does not require any adjustable parameter. Test calculations on the NiF₆^4? system are reported and compared with experimental and ab initio results.     

Density matrix methods in orbital optimization for MCSCF calculations

The problem considered is that of selecting the finite orbital basis which will minimize the energy in a given size CI calculation. (1) A one-body operator is defined which has as eigenfunctions the desired optimal basis. The operator is defined in terms of the basis which leads to a self-consistency problem of Hartree-Fock type. (2) A method of successive orbital rotations is defined which is shown to have desirable convergence properties.     

Self-consistent molecular orbital calculations on organoboron compounds

The Pariser-Parr-Pople method has been used to calculate the electronic structures and spectra of three methylvinylboranes. Two separate calculational models of the methyl group are considered and the agreement between observed and calculated spectral quantities is good. The reorganisation energies of these molecules are also evaluated.

---

Zusammenfassung Mit der Methode von Pariser, Pople und Parr werden Struktur und Spektren der -Elektronensysteme von drei Methylvinylboranen berechnet, ebenso ihre Reorganisationsenergien. Für die Methylgruppe werden zwei verschiedene Modelle verwendet. Die übereinstimmung zwischen berechneten und experimentellen Werten ist gut.

---

Résumé A l'aide de la méthode Pariser-Parr-Pople nous avons calculé les structures et les spectres -électroniques de trois méthylvinylboranes. Nous considérons deux modèles du groupe méthyl; les spectres observés et calculés s'accordent bien. En plus, les énergies de réorganisation de ces molécules sont évaluées.

---

---

One of us (D.R.A.) wishes to thank the S.R.C. for a maintenance grant.     

Self-consistent molecular orbital calculations on organoboron compounds

The PPP-SCMO-method is used to calculate the energies of the ground and lower excited states of triphenylborane and tri-p-tolylborane for which both planar and propellor models are considered. It is found necessary to include a large measure of configuration interaction in the excited states in order to produce satisfactory agreement with published u.v. spectra. However the calculations are not diagnostic for the detailed geometry of these compounds. The electronic spectra may be interpreted in terms of ring exciton states and interring and phenyl-boron charge-resonance states. The electron density on the central atom and the B-C bond orders are small and so the phenyl rings are but little perturbed by their bonding to boron.

---

Zusammenfassung PPP-SCMO-Rechnungen wurden für Triphenyl- und Tri-p-tolylboran durchgeführt. Ohne ausgedehnte Konfigurationenwechselwirkung erhält man keine befriedigende Übereinstimmung mit veröffentlichten UV-Spektren. Die Deutung der Elektronenspektren geschieht mit Hilfe der Begriffe der einzelnen Einganregung sowie des Ring-Ring- und des Ring-Bor-Ladungsaustausches. Ob die Molekeln in einer planaren oder einer Propeller-Konformation vorliegen, kann aufgrund dieser Rechnungen nicht entschieden werden. Bor--Ladung und B-C--Bindungsordnung sind gering; die Phenylringe werden durch ihre Bindung an Bor kaum beeinflußt.

---

Résumé La méthode SCMO-PPP a été utilisée pour calculer les énergies de l'état fondamental et des plus bas états excités du triphènylborane et du tri-p-tolylborane pour lequel on a considéré les modèles plan et en hêlice de bâteau. L'inclusion d'une interaction de configurations étendue entre états excités est nécessaire pour obtenir un accord satisfaisant avec les spectres UV publiés. Mais les calculs ne permettent pas de préciser la géométrie détaillée de ces composés. Les spectres électroniques peuvent être interprétés en termes d'états excitoniques du cycle et d'états de résonance de charge entre cycles et entre le phényl et le bore. La densité électronique sur l'atome central et les indices de liaison sur les liaisons B-C sont faibles ce qui montre que les noyaux phényliques sont peu perturbés par leur liaison au bore.

     

Self-consistent molecular orbital calculations on organoboron compounds

The electronic structures of three dihalogenophenylboranes have been investigated by the Pariser-Parr-Pople method. The agreement between the observed and calculated electronic spectra is quite good.

---

Zusammenfassung Die -Elektronen-Strukturen von drei Dihalogenphenylboranen wurden mit der PPP-Methode untersucht. Beobachtete und berechnete Elektronenspektren stimmen recht gut überein.

---

Résumé On a étudié par la méthode de Pariser-Parr-Pople les structures électroniques de trois dihalogénophenylboranes. L'accord entre les spectres électroniques observés et calculés est assez satisfaisant.

     

Ab initio molecular orbital calculations for butadiene and its ions

Ab initio calculations are reported for cis and trans butadiene and some of their ions. The calculations are compared with semi-empirical results, and used to predict coupling constants.     

Comparison of the numerical stability of methods for anharmonic calculations of vibrational molecular energies

On model examples, we compare the performance of the vibrational self-consistent field, variational, and four perturbational schemes used for computations of vibrational energies of semi-rigid molecules, with emphasis on the numerical stability. Although the accuracy of the energies is primarily dependent on the quality of the potential energy surface, approximate approaches to the anharmonic vibrational problem often do not converge to the same results due to the approximations involved. For furan, the sensitivity to variations of the anharmonic potential was systematically investigated by adding random noise to the cubic and quartic constants. The self-consistent field methods proved to be the most resistant to the potential variations. The second order perturbational techniques are sensitive to random degeneracies and provided the least stable results. However, their stability could be significantly improved by a simple generalization of the perturbational formula. The variational configuration interaction is practically limited by the size of the matrix that can be diagonalized for larger molecules; however, relatively fewer states need to be involved than for smaller ones, in favor of the computing.     

Second-row molecular orbital calculations: II. Semi-empirical calculations of dipole moments

The ability of four semi-empirical methods to predict dipole moments of molecules containing atoms in the second row of the periodic table is investigated. None of the methods is capable of consistently reproducing either magnitudes or qualitative trends; however, the CNDO method of Santry gives the best agreement overall. The original CNDO method of Santry and Segal emphasizes the importance of d orbitals to a greater extent than does the Santry method. Comparisons are presented with non-empirical results when possible.     

Second row molecular orbital calculations: III. Semi-empirical calculations of geometries

The ability of four semi-empirical methods to predict geometries of molecules containing atoms in the second row of the periodic table is investigated for about 80 molecules. Non-empirical, minimal basis set calculations, with and without optimization of valence orbital exponents, are carried out for a number of diatomic molecules. While none of the methods are capable of predicting geometries with an accuracy comparable to the first row parametrization, the SPD' method of Santry and the related INDO method of Benson and Hudson appear to be the most consistent. The ab initio calculations do not suffer from the drawbacks exhibited by the latter two semi-empirical methods. From this it is concluded that the failure of such methods lies in the parametrization rather than in the use of a minimal basis set.     

Standard bond lengths for use in ab initio molecular orbital calculations

Standard bond lengths are proposed for a wide variety of bond lengths involving first row elements. These were obtained as average values from a large number of calculations made at the ab initio molecular orbital 4-31G level with geometry optimization. It is shown that these are generally in good agreement with accurate experimental values, where available.