An approximate ab initio method based on the DIM model

2000 ◽  
Vol 78 (12) ◽  
pp. 1575-1586 ◽  
Author(s):  
John M Cullen
Keyword(s):  
Ab Initio ◽  
Excited States ◽  
Electron Correlation ◽  
Correlation Energy ◽  
Basis Sets ◽  
Ab Initio Method ◽  
First Order ◽  
Hartree Fock ◽  
Modified Method ◽  
Electron Correlation Energy

Using a second quantized formulation, an approximate diatomics in molecules (DIM) theory is presented in which all three- and four-centered electronic integrals are neglected. To ameliorate the effects of this approximation, the DIM one electron operator is constructed so that the true ab initio first-order density matrix and total energy are reproduced at the Hartree–Fock level. The resulting model was extensively tested for a variety of basis sets for its capability of capturing both the dynamic and nondynamic components of the electron correlation energy as well as the energies of excited electronic states. A modified method in which the DIM one-electron operator is formed from the initial extended Hückel guess of the Hartree–Fock orbitals was also found to produce excellent results.Key words: DIM, electron correlation energy, excited states, semiempirical.

On the performance of molecular model core potential orbitals in spin-orbit and electron correlation studies

1996 ◽  
Vol 74 (6) ◽  
pp. 1248-1252 ◽  
Author(s):  
Dietmar Krause ◽  
Mariusz Klobukowski
Keyword(s):  
Electron Correlation ◽  
Correlation Energy ◽  
Molecular Model ◽  
Virtual Space ◽  
Coupling Constants ◽  
Basis Sets ◽  
Spin Orbit ◽  
Hartree Fock ◽  
Electron Correlation Effects ◽  
Model Core Potentials

The role of improved parametrization and accurate basis sets in model core potentials was studied in calculations of the spin-orbit coupling constants (in PH, AsH, and SbH) and of the electron correlation effects (in P2, As2, and Sb2). An effective method of identifying and removing the intruder quasi-core orbitals from the virtual orbital space was proposed in connection with the post-Hartree–Fock calculations. The results demonstrated that (a) the flexible valence basis sets allow evaluation of the spin-orbit effects without resorting to any scaling techniques and (b) the intruder quasi-core orbitals, even if left imbedded in the virtual space, have negligible effect on the values of the electron correlation energy. Key words: effective core potentials, basis sets, spin-orbit effects, electron correlation.

scholarly journals THE METHOD OF INCREMENTS — A WAVEFUNCTION-BASED AB-INITIO CORRELATION METHOD FOR SOLIDS

2007 ◽  
Vol 21 (13n14) ◽  
pp. 2204-2214 ◽  
Author(s):  
BEATE PAULUS
Keyword(s):  
Experimental Data ◽  
Ab Initio ◽  
Ground State ◽  
Infinite System ◽  
Correlation Energy ◽  
Correlation Method ◽  
Many Body ◽  
Hartree Fock ◽  
The Many ◽  
Good Agreement

The method of increments is a wavefunction-based ab initio correlation method for solids, which explicitly calculates the many-body wavefunction of the system. After a Hartree-Fock treatment of the infinite system the correlation energy of the solid is expanded in terms of localised orbitals or of a group of localised orbitals. The method of increments has been applied to a great variety of materials with a band gap, but in this paper the extension to metals is described. The application to solid mercury is presented, where we achieve very good agreement of the calculated ground-state properties with the experimental data.

Relativistic AB Initio calculations of the properties of ionic solids

1986 ◽  
Vol 320 (1553) ◽  
pp. 107-158 ◽  
Keyword(s):  
Ab Initio ◽  
Short Range ◽  
Correlation Energy ◽  
Electron Gas ◽  
Empirical Method ◽  
Coulombic Interactions ◽  
Ionic Solids ◽  
Electron Correlation Energy ◽  
Crystal Properties ◽  
Semi Empirical

Relativistic ab initio calculations of inter-ionic potential energies are used to develop a reliable non-empirical method for predicting the properties of ionic solids containing the heaviest ions. A physically realistic method for describing the non-negligible differences between free and in-crystal ion wavefunctions is described. Functions are presented for describing the partial quenching, arising from ion wavefunction overlap, of the standard long-range form of the inter-ionic dispersive attractions. These attractions are shown to be distinct from the contributions to the inter-ionic potentials that arise from that portion of the electron correlation energy which is nonzero solely because of overlap of the ion wavefunctions. The results presented for NaCl, MgO and the fluorides of Li, Na, Ag and Pb show that these modifications overcome the deficiencies of previous calculations. Ab initio predictions of the closest cation-cation and anion-anion short-range interactions, which are not available from semi-empirical fits to experimental data, are presented. The non-point coulombic interactions between pairs of anions, derived by adding the dispersive attractions to the short-range interactions, are compared with previous semi-empirical and approximate ab initio results. The uncorrelated short-range inter-ionic potentials computed exactly are compared with those predicted from electron-gas theory. The use of the electron-gas approximation to describe any of these potentials degrades the quality of the predicted crystal properties.

scholarly journals QM AND AB INITIO INVESTIGATION ON THE HYDROGEN BONDING, NMR CHEMICAL SHIFTS AND SOLVENT EFFECTS ON THE DPPE

2010 ◽  
Vol 7 (3) ◽  
pp. 260-272
Author(s):  
M. Monajjemi ◽  
A. Nouri ◽  
H. Monajemi
Keyword(s):  
Hydrogen Bonding ◽  
Ab Initio ◽  
Atomic Orbital ◽  
Theoretical Data ◽  
Basis Sets ◽  
Stable Complex ◽  
Head Group ◽  
Water Molecules ◽  
Hartree Fock ◽  
Zwitterionic Form

The hydrogen bonding effects that were produced from interaction of membrane lipid dipalmitoylphosphatidyl-ethanolamine (DPPE) with 1-5 water molecules, has been theoretically  investigated through the quantum mechanical calculations at the Hartree-Fock level of theory and the 3-21G, 6-31G and 6-31G* basis sets with the computational package of Gaussian 98. According to the obtained results of the structural optimization of the isolated DPPE in the gas phase, we can see the evidences of interactions in the head group of this macromolecule (from the molecular point of view we have a proton transfer from the ammonium group to the phosphate oxygen of zwitterionic form. As we know that the hydrogen bonding of DPPE with water molecules which have surrounded its head group plays an important role in the permeability of DPPE. So, in order to understand the microscopic physico-chemical nature of this subject we have analyzed bond and torsion angles of DPPE before and after added water molecules.  In this paper we have theoretically studied the complexes DPPE with water molecules which have surrounded its head group. As mentioned before, this theoretically study has been done through Hartree-Fock level of theory by using simple basis sets. Theoretical data shows that the interaction of head group of DPPE with water molecules causes some changes in the geometry of DPPE which were explained by the contribution of zwitterionic form of DPPE macromolecule, and finally hydrated DPPE becomes stable complex. Comparison between theoretical and experimental geometry data of DPPE macromolecule shows that the calculation at the HF/3-21 level of theory produces results which they are in better agreement with the experimental data. Moreover the hydrogen bonding effects on the NMR shielding tensor of selected atoms in the hydrated complexes of DPPE were reported. The ";Gauge Including Atomic Orbitals"; (GIAO) approaches within the SCF-Hartree-Fock approximation have been used in order to investigate the influence of hydrogen bonding of DPPE-water complex on the shielding tensors. Finally, the solvent affects on the stability of DPPE macromolecule, dipole moment and atomic charge of some selected atoms of DPPE molecule was discussed using Onsager model and Merz-Singh-Kolman schema.   Keywords  : Gauge Including Atomic Orbital, DPPE, hydrogen bonding, solvation, quantum mechanics, ab initio

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