It is demonstrated that semiempirical methods give electrostatic potential (ESP) derived atomic point charges that are in reasonable agreement with ab initio ESP charges. Furthermore, we find that MNDO ESP charges are superior to AM1 ESP charges in correlating with ESP charges derived from the 6-31G* basis set. Thus, it is possible to obtain 6-31G* quality point charges by simply scaling MNDO ESP charges. The charges are scaled in a linear (y = Mx) manner to conserve charge. In this way researchers desiring to carry out force field simulations or minimizations can obtain charges by using MNDO, which requires much less computer time than the corresponding 6-31G* calculation.

Atomic charges derived from semiempirical methods

Publisher Summary This chapter discusses a model that relies on the knowledge of the molecular electrostatic potential, which is derived from a molecular wavefunction by using the usual methods for calculating the mean expectation value of an operator. In its basic premises the model employs quantum mechanics, with only the approximations necessary in molecular quantal calculations. The model is also discussed regarding its relationships with the Hellmann–Feynman theorem. The electrostatic potential V itself is examined in order to show how the electrostatic potential reflects the characteristics of the electronic distribution of a molecule and then the reliability of V is discussed as a reactivity index. The shape of the electrostatic potential and its relationship to the electronic molecular structure is discussed with the aid of various examples. One of them includes the glycine tautomers and the corresponding anion example. The chapter also discusses the electrostatic molecular potential in terms of local group contributions.

Electronic Molecular Structure, Reactivity and Intermolecular Forces: An Euristic Interpretation by Means of Electrostatic Molecular Potentials

A hierarchy of three models for combined quantum mechanical (QM) and molecular mechanical (MM) approaches is presented. They simplify the QM description of large molecules by reducing it to the electronically important fragment which interacts with the molecular mechanically treated remainder of the molecule. In the simplest model A, the QM fragments are only mechanically embedded in their MM environment. The more refined models B and C include a quantum mechanical treatment of electrostatic interactions between the fragments and a semiclassical description of polarization. The implementation of models A-C for MNDO type wavefunctions and the MM3 force field is outlined. Selected applications in organic chemistry are discussed, addressing the ability of the proposed models to reproduce substituent effects (MM) on chemical structure and reactivity (QM). These applications include protonations, deprotonations, hydride transfer reactions, nucleophilic additions, and nucleophilic ring cleavage reactions.

Hybrid Models for Combined Quantum Mechanical and Molecular Mechanical Approaches

The multiconfiguration self-consistent field (MCSCF) method offers the most general approach to the computation of chemical reactions and multiple electronic states. This review discusses the design of MCSCF wavefunctions for treating these problems and the interpretation of the resulting orbitals and configurations. In particular, localized orbitals are convenient both for selection of the appropriate active space and for understanding the computed results. The computational procedures for optimizing these wavefunctions and the techniques for recovery of dynamical correlation energy are reviewed.

The construction and interpretation of MCSCF wavefunctions.

The use of nmr methods for conformational studies of nucleic acids

The possibilities of utilization of CNDO wave functions for computing molecular electrostatic potentials are studied by comparison with ab initio results for H2O and H2CO.

Molecular electrostatic potentials: Comparison of ab initio and CNDO results

A semi-empirical self-consistent field calculation has been carried out for the purine and pyrimidine bases of the nucleic acids after an appropriate optimization of the integral values using reference compounds. Special emphasis has been laid upon results concerned with the ionization potentials, electron affinities, spectral transitions, triplet states, dipole moments and basicities of the compounds. They enable in particular a detailed interpretation of both the individual ultra-violet absorption spectra of the bases and of their interrelations.

Theoretical study of the electronic properties of the purine and pyrimidine components of the nucleic acids

Theoretical study of the electronic properties of biological purines and pyrimidines. II. The effect of configuration mixing

The CNDO/2 and INDO approximations (with their original parametrization) are utilized for the calculation of transition energies. The effect of including all (σ and π) singly excited configurations is assessed in C2H4, H2CO, HCOOH and HCONH2, and the results are compared to experimental transitions and to the available non-empirical calculations. The effect of extensive mixing is then considered in larger molecules.

The effect of σ and π singly-excited configurations in the calculation of excited states by the CNDO and INDO approximations

The approximation made in the calculation of molecular dipole moments by including only the point charges and the atomic dipoles is evaluated in different all-valence (or all)-electrons MO procedures. In the CNDO method, the use of the exact formula after retransformation of the atomic basis into Slater orbitals gives poorer values than the Pople-Segal's procedure.

Claude Giessner-Prettre

Papers

Molecular electrostatic potentials: Comparison of ab initio and CNDO results

Theoretical study of the electronic properties of the purine and pyrimidine components of the nucleic acids

Theoretical study of the electronic properties of biological purines and pyrimidines. II. The effect of configuration mixing

The effect of σ and π singly-excited configurations in the calculation of excited states by the CNDO and INDO approximations

On the calculation of molecular dipole moments