Calculation of effective atomic-charges from molecular orbital wavefunctions

Abstract

W.2Mulliken's method for obtaining gross atom-charges has been modified by use of a partitioning of the overlap densities which preserves their dipole moment. The gross atom charges are now determined by the elements of (Pg), rather than (PS), where 5 is a non-symmetric matrix whose elements depend upon the various overlap integrals, internuclear distances, and centroids of the overlap densities. The method is applied to the ground-states of some first-row diatomic molecules. For comparison, contours of the one-electron density function, and a difference density function, are presented for BH and FH. The effective atomic charges and contour diagrams are both calculated by use of two different sets of atomic basis functions. The calculated effective atomic charges are sensitive to both the choice of basis functions and the method used for partitioning the overlap densities.THE population analysis of LCAO molecular orbital wavefunctions was originally proposed by Mulliken to provide a means of extracting useful chemical information from calculated molecular wave-functions. Mulliken also described the calculation of gross atomcharges, or effective atomic charges, from the elements of the atom bond-population matrix, P, which is constructed from the atomic-orbital coefficients of the occupied molecular-orbitals. Following this work, it has now become widespread practice, in both semi-empirical and ab initio calculations, to derive a set of effective atomic-charges from the calculated molecular-orbital wave-function. These effective atomic-charges are then frequently used to discuss the extent of charge transfer within molecules, but there have been few attempts to This paper presents the results of a population analysis on the ground-states of some first-row diatomic molecules : two methods for estimating the effective atomic charges were used. The results for BH and FH are compared with the corresponding density-functions, P( 1,l). Finally, difference density-functions are plotted for these molecules to show more clearly how the electron distribution changes on molecule formation.

Determining Partial Atomic Charges for Liquid Water: Assessing Electronic Structure and Charge Models

“…Among other issues, these orbital-based methods are often overly-sensitive to basis set. [45][46][47][48] NPA is more stable to changes in the basis set than Mulliken, 47 but NPA also tends to over-estimate the magnitude of the charges. 47,48 Charge models in Class III determine charges based on analysis of some physical observable from quantum-chemical calculations, such as the electrostatic potential (ESP) or the electron density.…”

A consistent semiempirical theory for the calculation of ground and excited state properties

“…In order to see this clearly, we first consider the relationship between rr electron and all valence electron theories in more detail. For planar molecules, the full Fock operator may be rigorously written in the form (2) where h is the core Hamiltonian, J is a Coulomb operator, and K is an exchange operator. The PPP formalism collapses the a electrons into the nuclei, producing a rigid skeleton in which the carbon atom has a core charge of unity.…”

Determining Partial Atomic Charges for Liquid Water: Assessing Electronic Structure and Charge Models

“…Among other issues, these orbital-based methods are often overly-sensitive to basis set. [45][46][47][48] NPA is more stable to changes in the basis set than Mulliken, 47 but NPA also tends to over-estimate the magnitude of the charges. 47,48 Charge models in Class III determine charges based on analysis of some physical observable from quantum-chemical calculations, such as the electrostatic potential (ESP) or the electron density.…”