Semiempirical wavefunctions

Usually we need more c s than the small basis set of an extended Hiickel or other semiempirical calculation supplies a projected semiempirical wavefunction is then used, with the missing c s extrapolated from the available ones). Using these c s and Eq. 5.121 = 5.81 we calculate the initial-guess P s for Eqs. 5.106-5.108 since there is only one occupied MO (n = 1 in Eq. 5. 121) the summation has only one term ... 

Krack M, Koster AM, Jug K. Approximate molecular electrostatic potentials from semiempirical wavefunctions. J Comput Chem 1997 18 301-312. 

Equation [4] is an exact formula for the electrostatic potential due to a set of nuclei (Z jj and an electronic density p(r). The latter is generally obtained computationally from an ab initio or semiempirical wavefunction or, more recently, from a local density functional procedures. p(r) is therefore necessarily approximate it follows that the resulting V(r) is also approximate. 

Stewart J J P 1989 Optimization of parameters for semiempirical wavefunctions J. Comput Chem. 10... 

Electrostatic potentials are important tools in modeling because they indicate the preferred points of nucleophilic and electrophilic attack in large molecules. Their evaluation is straightforward at the ab initio level, and RHF/6-31G potentials are established as the current de facto standard in the field. For semiempirical wavefunctions, there are two traditional procedures for calculating electrostatic potentials The quasi ab initio approach [149] transforms the wavefunction to a deorthogonal-ized basis and then computes the potential analytically [7,150-153] whereas the purely semiempirical approach [7,153-156] evaluates the... 

Approximate expression of the one-electron integrals are sometimes employed, especially in connection with semiempirical wavefunctions. The first proposal was made by Carbo and Martin [46], and several other variants have been suggested over the years. 

Besler, Merz and Kollman [163] examine PD-AC values obtained with the MNDO and AM 1 methods (using the procedure of deorthogonalization of the semiempirical wavefunctions) and compare them with STO-3G and 6-3IG values. The authors find that MNDO charges are superior to AMI charges in correlating with PD-AC values obtained from 6-3IG wavefunctions. The analysis is with 20 compounds, and the fitting exploits the Lagrangian multipliers method. 

Field M J 1991 Constrained optimization of ab initio and semiempirical Hartree-Fock wavefunctions using... 

Luque, F. J., F. Illas, and M. Orozco. 1990. Comparative Study of the Molecular Electrostatic Potential Obtained from Different Wavefunctions. Reliability of the Semiempirical MNDO Wavefunction. J. Comp. Chem. 11,416. 

We now turn to the problem of simplifying the recovery of the dynamic correlation energy. We consider the simplest situation, viz., where the zeroth-order wavefunction can be chosen as the SCF approximation. A challenging disparity exists between the energetic smallness of these refinements and the complexity and magnitude of the computational efforts required for their variational determination. In order to reduce this disproportion, various semiempirical approaches have been proposed (56-61), notably in particular the introduction of semiempirical elements into MP2 theory which has led to the successful Gn methods (62-64). 

We have learned about the unique ordering of the carbon net charges relative to one another. All methods using Mulliken s population analysis, both ab initio and semiempirical, no matter what basis sets are used to construct the wavefunctions, reproduce the following sequence of inductive effects ... 

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