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The band structure of poly( para -phenylene-vinylene) (PPV) has been calculated. A Hartree–Fock (HF) crystal orbital calculation served as a starting point for a quasi-particle (QP) method to take the electron correlation into account. A double zeta basis set was used with polarization functions. The calculated ionization potential (using Koopmans' theorem) is somewhat larger than the experimental one and the width of the correlation corrected valence band is in quite good agreement with the value obtained from previous calculations. On the other hand, although the correlation correction decreased the fundamental gap substantially as compared with its HF value (from 7.42 to 4.89 eV), it is still twice as large as its directly measured value. The main reason for this large disagreement is that we have calculated a single chain of PPV, while the measured value refers to a layer. Looking at the crystal structure of PPV one finds that it is closely packed with the smallest interchain distance of ∼2.5 A. In such case no Madelung potential can be used to describe the rest of the crystal around the calculated chain. For this reason we plan different two-dimensional calculations to resolve the contradiction concerning the fundamental gap.

The method of separated molecular orbitals (SMO) has already proved to be useful for describing the energy partitioning at the Hartree–Fock (HF) level in several weakly interacting systems. In the present paper, the total energy decomposition into different contributions according to the interacting units is discussed at the HF level for a zeolite model system. The effect of the basis set superposition is considered by the use of SMO in a simple and straightforward manner.

The correlation-corrected band structures of quasi 1D poly(para-phenylene) (PPP) and poly(peri-naphthalene) (PPN) are presented, using the ab initio quasi-particle method based on the iterative solution of the inverse Dyson equation. A double zeta basis set is used with polarization functions for PPN and a double zeta basis is used for PPP.

The results presented in this paper for the dimer of H2O as well as for the dative complexes formed by the interaction of BH3 + H2O and AlH3 + H2O suggest that about 50% of the dispersion energy can be recovered, using a 6-31G(d) basis set, in cases when only the contributions of those localized molecular orbitals are included which are in the vicinity of the interaction region. The reduction in computing time is about an order of magnitude. The procedure applied makes it possible to avoid basis set superposition errors at the correlated level. The method presented opens the way for studying large clusters containing various atoms beyond the HF level.

The dispersion interaction energy with the molecules BH3 and NH3 was computed using a specific supermolecule method. The essence of the approach used lies in applying a localized representation in the virtual space. This method makes it sensible to investigate the above systems surrounded by several water molecules.

An ab initio self-consistent field (SCF) method with standard STO-3G and 6-31G ∗ basis sets was used to study some normal saturated hydrocarbons. The occupied and virtual canonical orbitals were localized separately by Boys' procedure and the correlation energy corrections were calculated in the localized representation of the many-body perturbation theory. Introducing the concept of “neighbourhood order” the local and non-local effects can be separated and the correlation energy corrections can be partitioned accordingly. It was found that the correlation energy corrections in these molecules are predominantly of local origin. The contributions to the pair correlation energies are transferable to a good approximation. In analysing the contributions of the localized virtual orbitals, some interesting structural features were identified.

The ab initio SCF method was applied to the all-trans polyacetylenes C 2 n + 2 H 2 n + 4 ( n = 0, 2, 4, 6, 8, 10), by using standard STO-3G basis set. The localized molecular orbitals obtained by Boys' procedure were studied and compared. It is shown that, in spite of the “tails”, the contributions of the localized molecular orbitais (orbital energy, charge distribution, effective volume etc.) on increasing the length of the molecule converge to definite values, depending upon the position of the corresponding localized orbital in the molecule. It is shown that the difference with respect to the transferability between the aliphatic hydrocarbons and the all-trans conjugated polyenes is only quantitative. In the latter compounds the effects of the fourth neighbours should also be included.

Recent experimental sind theoretical studied reporting remarkable monobridged structures for Si 2 H 2 , Al 2 H 2 , and Ga 2 H 2 have motivated us to re-investigate the singlet potential energy surface of Ge 2 H 2 using the self-consistent-field (CCP), single and double excitation configuration interaction (CISD), and single and double excitation coupled cluster (CCSD) methods in conjunction with a double-ζ plus polarization basis set (DZP). In addition to the dibridged (or butterfly) ground state and the low-lying vinylidene-like minimum reported earlier by Grev and DeLeeuw, our study predicts the existence of a monobridged isomer, which is characterized as a minimum by means of harmonic vibrational frequency analyses

The ionization potential of the closed shell H2CO, H2COH+, H2COH22+ as well as the proton affinity of H2CO and H2COH+ have been studied at a variable CO bond length in the ground singlet and some of the low lying excited triplet and ionized doublet states.

An ab initio SCF method has been applied to normal saturated hydrocarbons C2n+1H4n+4 (n=0,1,2,3,4), by using a standard STO-3G basis set. The localized orbitals of the molecules obtained by Boys'procedure are investigated and compared. It is shown that, in spite of the “tails” the contributions of the localized orbitals (orbital energy, charge distribution, effective volume, etc.), by increasing the length of the system, converge to definite values depending only upon the position of the corresponding localized orbital in the molecule. This offres a possibility of explaining the transferability of certain molecular properties in a natural way.

In a series of papers a new method which uses the molecular symmetry for treatment of extended molecules is investigated. In this paper it is shown that in the presence of model geometries the method — on which a new ab initio program named SYCETY is based — is effective for calculation of large, related systems.

An ab initio SCF method with a standard STO-3G basis set was used to study normal saturated hydrocarbons C 2 n +1 H 4 n +4 ( n =0,1,2,3,…). The occupied and the virtual canonical orbitals were localized separately by Boys'procedure and the second-order correlation energy was calculated in the localized representation of the MBPT. In order to separate local and non-local effects the concept of “neighbourhood order” was introduced and the correlation energies were partitioned according to this. It was shown that: (1) by taking into account only the zero-, first- and second-order “neighbourhoods”, more than 99% of the total second-order correlation energy could be recovered; (2) the contributions to the pair correlation energies were transferable to a good approximation; and (3) by increasing the system, the contributions of the most important pair correlation energies converged to definite values, depending only upon the position of the corresponding LMO in the molecule.