A modified PCILO method

The perturbative configuration interaction using strictly localized molecular orbitals, called the modified PCILO method, for which the use of the Rayleigh-Schrödinger many-body perturbation theory with the Moller-Plesset Hamiltonian partitioning is characteristic, has been proposed in this communication. On the CNDO/2 and INDO levels of Hamiltonian approximations strictly localized molecular orbitals have been constructed by solving modified Roothaan equations. From the zero and second order energy interatomic distances and harmonic force constants for some diatomic molecules have been calculated. The linear dependence of the correlation energy on the number of valence electrons in the series of the molecules CH₄, CH₃F, CH₂F₂, CHF₃ and CF₄ is perfect.     

Calculation of systems involving strong delocalized bonds within a modified PCILO framework. I. Valence-bond PCILO method

In describing mesomer structures by the original PCILO method several problems may occur because of the use of localized bonds. A so-called VB-PCILO including a superposition of several mesomer boundary structures in the CI perturbation treatment will be proposed.     

An improved CNDO/2 standard parametrization for bromine containing molecules

Bromine parameters are proposed, matching well the standard values of first and second row atoms, as introduced by Pople et al. This is shown by comparisons of geometries, configurations, conformations, and dipole moments, obtained with different approaches. The new parameter set was found in a very simple and efficient way that may also be useful for other tasks, e.g. in non-empirical calculations.     

Crystal calculation using the PCILO method

The PCILO method is used for the study of molecular systems with translational symmetry in a two-dimensional lattice. The calculation for the whole crystal is reduced to the calculation of unit cell pairs. By using the translational symmetry and the first-neighbour approximation, one shows that only four unit cell pairs have to be considered. The procedure described yields the ground-state energy and the charge distribution of the unit cell.     

Optimization of molecular geometries in the PCILO and CNDO/2 formalism. The barrier to internal rotation in thioacetaldehyde

The barrier to internal rotation in thioacetaldehyde was investigated within the PCILO and CNDO/2 framework using standard and optimized geometries. The optimized geometries give for the barrier of PCILO a value closer to the experimental one (1.06 kcal mol^?1) whereas the CNDO/2 results yield a qualitatively wrong conformational behavior.     

Calculations for para-substituted phenols using the CNDO/2 method

Theoretical and Experimental Chemistry -     

Assignments in photoelectron spectroscopy using the CNDO/2 method

In the photoelectron spectra of molecules vibrational fine structure often accompanies the observed ionization potentials (IPs). For non-diatomic molecules it is difficult to make vibrational assignments because the frequencies of the ion often differ greatly from those of the neutral molecule. In order to make both the vibrational assignments and the assignments of the IPs to molecular orbitals (MOs) using Koopmans' theorem CNDO/2 MO calculations have been carried out. Within the framework of this method a quantity E _AB has been calculated which is the change in potential energy between an atom-pair A-B for a particular MO on ionization. Application to some diatomic molecules has shown that this quantity reflects the changes in vibrational frequencies on ionization. The IP-MO assignments are made such that removal of an electron from a particular MO is in accord with the observed vibrational fine-structure; attempting to make these assignments on the basis of the calculated MO ordering alone has been shown to be very unsatisfactory.

---

Zusammenfassung Photoelektronenspektren von Molekülen zeigen neben den Ionisierungspotentialen oft noch eine Schwingungsfeinstruktur, deren Zuordnung bei nicht-zweiatomigen Molekülen wegen der starken Frequenzverschiebungen vom Molekül zum Ion Schwierigkeiten macht. Um sowohl Schwingungsais auch Ionisierungs-Zuordnungen vornehmen zu können, wurden Rechnungen mit dem CNDO/2-Verfahren durchgeführt. Dabei stützt man sich auf die Änderung der potentiellen Energie zwischen dem Atompaar AB in bezug auf ein bestimmtes MO bei Ionisierung (E _AB). Die Anwendung auf zweiatomige Moleküle zeigt, daß diese Größe tatsächlich die Änderung der Schwingungsfrequenzen wiedergibt, und die Zuweisungen des IP werden so vorgenommen, daß die Entfernung eines Elektrons aus einem MO im Einklang mit der Schwingungsfeinstruktur ist. Dagegen erweist sich die einfache Zuweisung auf Grund der MO-Reihenfolge als schlechter.

     

Efficient parametrization of complex molecule–surface force fields

We present an efficient scheme for parametrizing complex molecule–surface force fields from ab initio data. The cost of producing a sufficient fitting library is mitigated using a 2D periodic embedded slab model made possible by the quantum mechanics/molecular mechanics scheme in CP2K. These results were then used in conjunction with genetic algorithm (GA) methods to optimize the large parameter sets needed to describe such systems. The derived potentials are able to well reproduce adsorption geometries and adsorption energies calculated using density functional theory. Finally, we discuss the challenges in creating a sufficient fitting library, determining whether or not the GA optimization has completed, and the transferability of such force fields to similar molecules. © 2015 Wiley Periodicals, Inc.     

π-Molecular complexes: A modified CNDO study of the benzene-borazine system

The standard CNDO/2 method is shown to be unable to produce meaningful potential curves for π-π-type molecular complexes. A modification of this method in which pairs of atoms associated with the same molecule and with different molecules are differentiated leads to reduced intermolecular bonding and provides reasonable stabilization energies and intermolecular separations. Calculations based on this modified method indicate that benzene-borazine and borazine-borazine complexes in which the molecules are symmetrically disposed in parallel planes can exist in the ground state. The stabilization energies are calculated to be in the range 2–5 kcal/mole for benzene-borazine and 5–18 kcal/mole for borazine-borazine, with interplanar separations near 3 Å in both cases.     

PCILO and CNDO calculations on the specific interaction in two model analogs of BPA-polycarbonate and a linear aliphatic polyster

Previous investigations have given evidence of the existence of specific chemical interactions between poly(bisphenol-A carbonate) (PC) and a number of linear aliphatic polyesters, with the consequent formation of compatible polymer alloys. Here we address the particular kind of specific interaction that takes place in these systems by performing a quantum-mechanical study of the apparently most reasonable types of molecular interactions occurring in model compounds of the above polymers. Complete neglect of diffential overlap, (CNDO), in its CNDMEX version, and perturbative configuration using localized orbitals (PCILO) were the calculation schemes used on diphenyl carbonate (DPC) and dimethyl succinate (DMS), selected as models. It was found that, on a one-to-one basis, the DPC-DMS system is energetically more stable than the individual separate molecules; on examination of the possible interaction sites for the two chemical species, the interaction between the ester group oxygen in one DMS molecule with one of the phenyl rings in DPC appears to be the most favorable type of molecular interrelation, even though hydrogen bond formation is also possible between the oxygen atoms in DPC and the methylene hydrogens in DMS. (The former interaction decreases the total energy of the system by 0.12 kcal/mol, more than the latter.)     

Analytical calculation of the electrostatic interaction energy within the CNDO framework

This work analyses the adequacy of an analytical electrostatic energy formulation within the CNDO framework to predict the stable conformations of large molecular complexes. Comparisons are made with abinitio results for small systems such as water-formamide, methanol-water-imidazole, or guanine-cytosine and with AM 1 results for two large systems: a molecular tweezer + the 9-methyladenine complex and a model active site of the α-chymotrypsin and its ligand complex. This approach is efficient in providing reliable confromers for large molecular systems in a very fast way. © 1993 John Wiley & Sons, Inc.     

A CNDO/2 calculation on stilbene

The equilibrium conformations of trans- and cis-stilbene and the rotational barrier between the two isomers were calculated using the CNDO/2 method. The results for the equilibrium angles and distances are in good agreement with experimental data. However, according to the calculations the phenyl rings would have a nearly free rotation which is only limited by steric hindrance. This result obviously deviates from the current view. The results are discussed in relation to previous theoretical and experimental evidence.

---

Zusammenfassung Die Gleichgewichtskonformation von trans- und cis-Stilben und die Rotationsbarriere zwischen zwei Isomeren wurde nach der CNDO/2-Methode berechnet. Die Ergebnisse in bezug auf die Geometrie stimmen mit experimentellen Daten gut überein, die Rotationsbarriere für die Drehung der Phenylringe dagegen ist praktisch Null.

---

Résumé La méthode CNDO/2 a été employée pour calculer les conformations d'équilibre du trans- et du cis-stilbène ainsi que la barrière de rotation entre ces deux isomères. Angles d'équilibre et distances calculés sont en bon accord avec les données expérimentales. Cependant les calculs indiquent que les cycles phényles devraient posséder une rotation libre soumise seulement à l'empêchement stérique. Ce résultat s'écarte de l'opinion courante. Les résultats sont discutés en relation avec les données théoriques et expérimentales antérieures.

---

---

This work was supported by grant No 2741-4 from The Swedish Natural Science Research Council and grant No 69-927/U 697 from The Swedish Board for Technical Development.     

A CNDO/S study on the alkylbenzene-TCNE complexes

The calculations based on the semiempirical CNDO/S Hamiltonian appeared useful in predicting the electronic structures of electron donor-acceptor (EDA) complexes of tetracyanoethylene (TCNE) and various alkylbenzenes. The results are in satisfactory agreement with the experimental data. The calculated complex stabilization energies are related to the experimental gas phase enthalpies of formation by linear regression (τ = 0.941). A fairly good correlation (τ = 0.985) between the measured and calculated energies of the charge transfer (CT) electronic transitions is established. The problem of double CT absorption bands is discussed by referring to the theoretical results. The ionization potentials of alkylbenzenes calculated by CNDO/S and MNDO methods are considered as well.     

A CNDO/2 study of the ethylene-chlorine reaction

CNDO/2 calculations have been carried out of the energies of reaction and changes in charge distribution for some reactions involving the ethylene chlorine complex. The addition of chlorine to ethylene may involve the attack of a chloride ion on the ethylene chlorine complex.     

Description of equilibrium bond lengths with the use of spectroscopic parametrization of the CNDO method including transition metals

Lensovet Leningrad Technological Institute. Plastpolimer Okhta Scientific-Production Association. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 4, pp. 22–28, July–August, 1991.     

A three‐dimensional AlIII/NaI metal–organic framework resulting from dimethylformamide hydrolysis

The three‐dimensional metal–organic framework poly[bis(dimethylammonium) [hexa‐μ₂‐formato‐κ¹²O:O′‐aluminium(III)sodium(I)]], {(C₆H₈N)₂[AlNa(HCOO)₆]}_n, was obtained serendipitously and has been characterized by X‐ray diffraction. The product has arisen as a result of a hydrolysis reaction of dimethylformamide (DMF) and contains dimethylammonium (DMA) cations included in structural voids formed by a three‐dimensional [AlNa(HCOO)₆]⁻ network. This study provides evidence that, in the presence of traces of aluminium, DMF stored in a glass bottle can be hydrolysed to formate and dimethylamine with simultaneous extraction of Na⁺ cations from the glass. It also demonstrates that care must be taken regarding the metal and water content when DMF is not freshly distilled, since the hydrolysis of amide can occur.