Crystal orbitals, Hartree-Fock calculation basis

Orbital-based cluster and periodic lattice calculations Non-periodic and periodic Hartree-Fock calculations (using basis sets, e.g. B3LYP, 6-3IG) GAUSSIAN CRYSTAL "... 

For the first example of fiilly ab initio periodic SCF study of a molecular crystal see Dovesi, R. Causa, M. Orlando, R. Roetti, C. Saunders, V.R. Ab initio approach to molecular crystals a periodic Hartree-Fock study of crystalline urea, J. Chem. Phys. 1990,92,7402-7411. The unit cell contains sixteen atoms and the basis set employed is 6-21G. The band structure, density of states, and deformation electronic densities are obtained. The lattice energy is -117 kJ mol without, and 67 kJmol with basis set superposition error correction [8]. The latter value reflects the absence of dispersion contributions in the HF approach (experimental heat of sublimation 88 kJ mol ). Such a study would now be comfortably feasible on any standard PC, but clearly the computational load of a periodic crystal calculation increases with the increasing number of atoms in the unit cell and size of the atomic orbital basis set. A good critical entry point to the literature is Spackman, M.A. Mitchell, A.S. Basis set choice and basis set superposition error (BSSE) in periodic Hartree-Fock calculations on molecular crystals, Phys. Chem. Chem. Phys. 2001,3,1518-1523. 

The electronic structures of poiy(fluoroacetylene) and poly(difluoroacetylene) have been investigated previously using the ab initio Hartree-Fock crystal orbital method with a minimum basis set (42). Only the cis and trans isomers with assumed, planar geometries were studied. The trans isomer was calculated to be more stable in both cases, and the trans compounds were predicted to be better intrinsic semiconductors and more conductive upon reductive doping than trans polyacetylene. However, our results show that head-to-tail poly(fluoroacetylene) prefers the cis structure and that the trans structure for poly(difluoroacetylene) will not be stable. Thus the conclusions reached previously need to be re-evaluated based on our new structural information. Furthermore, as noted above, addition of electrons to these polymers may lead to structural deformations that could significantly change the conductive nature of the materials. 

These results were in agreement with a previous calculation done using an ab initio Hartree-Fock crystal orbital method with a 7s/3p minimal basis set with the zW-trans geometry taken from the Extended Huckel calculation [12]. This method allowed complete... 

The calculation schemes usually used for point defect include 1. the choice of the model of the defective crystal 2. the choice of the Hamiltonian (Hartree-Fock, DFT or hybrid, semiempirical) 3. the choice of the basis for the one-electron Bloch functions decomposition hnear combination of atomic orbitals (LCAO) or plane waves (PW). 

In spite of all the evolution, there are still important problems for the calculation of intermolecular energies. Hartree-Fock (HF) methods use one-electron orbitals and therefore cannot account for those phenomena that depend on the simultaneous behavior of several electrons. Thus, HF energies may correctly represent the kinetic energies of electrons and the electrostatic effects between electrons and nuclei, but cannot take into account electron correlation. The results obtained at the limit of a complete (i.e. infinitely rich) basis set are called HF-limit energies and wavefunctions, the ideal best that can be obtained with one-electron orbitals. This intrinsic limitation forbids the treatment of dispersion energy, a crucial part of the intermolecular potential (see Chapter 4). Thus, for example, HF methods are intrinsically unsuitable for the calculation of the lattice energies of organic crystals. 

Due to the central role of DNA and proteins in biochemistry and biophysics the computation of the electronic structure of periodic polymers built from nucleotide bases, base pairs, nucleotides and amino acids, respectively, had been of high interest since about twenty years. Early calculations of the band structure of DNA related periodic polymers have been performed with the crystal orbital (CO) method on the basis of different semiempirical levels (1). Recently the results of ab initio Hartree-Fock CO (2, 3) band structure calculations for the four nucleotide base stacks (4-6), the two Watson-Crick base pair stacks (6), the sugar-phosphate chain (4,5) and the three nucleotides cytidine (4,5), adenylic acid and th3nnidine (6) have been reported. These computations represent a significant progress but the following improvements are required for a more accurate description of the electronic structure of real DNA and its transport properties ... 

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