Ghost atom

The concept of ghost-atoms only applies to Single Point calculations in the current version of HyperChem. 

Use Configuration Interaction to predict the electronic spectra of molecules. The Configuration Interaction wave function computes a ground state plus low lying excited states. You can obtain electronic absorption frequencies from the differences between the energies of the ground state and the excited states. 

A configuration interaction calculation is available only for single points when the reference ground state is obtained from an RHF calculation. 

The calculation mixes all single determinant wavefunctions that can be obtained from the ground state by exciting electrons from a subset of the occupied orbitals (of the ground state) to a subset of the unoccupied orbitals. The subsets are specified as a fixed number (highest occupied or lowest unoccupied) or by an energy criterion associated with the energy difference between the occupied orbital and the unoccupied orbital. 

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The representation of an essentially infinite framework by a finite SCF treated cluster of atoms, (with or without point-ions), inevitably leads to the problem of how to truncate the model-molecule . Previous attempts at this have included using hydrogen atoms l and ghost atoms . Other possibilities include leaving the electron from the broken bond in an open shell, or closing this shell to form an ionic cluster. A series of calculations were performed to test which was the host physically realistic, and computationally viable, solution to this problem for this system. 

LJ) potential (6). The diffusing atoms also have LJ forces between them. Atoms interact with a ghost atom in the substrate that is subjected to random and dissipative forces that closely match the forces exerted by a neighboring shell of atoms in the crystal. In this way the MD computation is limited to a relatively small number of mobile atoms and their ghost atoms, and the influence of the large number of atoms in the crystal is represented by the forces applied to the ghost atom. 

Many electronic structure programs, widely used to compute chemical shifts of atoms [61], can be used routinely to compute NICS employing ghost atoms at chosen points. The sign of absolute shieldings obtained in this manner are merely... 

Figure 7. GIAO-calculated shielding increment (Ao) of "ghost atoms" (triangles) and the McConnell equation-calculated shielding increment (circles) 2.0 A above ethene vs. lateral distance from the center of the carbon-carbon double bond.

To emphasize the physics again for the GLE-ghost atom technique, we note that the forces among the primary zone atoms are general and anharmonic. The coupling between the primary and ghost atoms is incorporated by the terms and cuoM T. The frictional force,... 

Before leaving the discussion of dissociative chemisorption results, we note a few further points. First, nearly all simulations have either ignored the effect of the lattice or included it via simple harmonic motion of a primary zone coupled via the GLE to ghost atoms. A recent investigation (Agrawal et al. 1987) showed that for the H2/Si(lll) system the lattice potential plays... 

Where E(XY) is the total energy of the XY base pair in the ground state E(Xxy) and E(Yxy) are the total energies of X (adenine in case of the AT (AU) base pair and guanine in the case of tiie GC base pair) and Y (thymine (uracil) in the case of the AT (AU) base pair and cytosine in the case of the GC base pair) monomeric moieties using the optimized XY base pair geometry and ghost atoms in place of the complementary base. 

Figure 24 a) Dependence of the BSSE on the number of neighboring atoms included in the CP correction for crystalline urea, with the LDA approximation (SVWN) and a 6-31G(d,p) basis set. (b) Urea molecule surrounded by a star of 63 neighboring ghost atoms. 

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