Atomic charge population

Despite th ese reservation s. Mu Ilikeri population -derived atomic charges arc easy to compute. Empirical investigation shows that they have various uses, they provide approximate representation of the 3D charge distribution within a molecule. 

The subscripts i and j denote two nuclei one in the QM region and one in the MM region. The atomic charges for the MM atoms are obtained by any of the techniques commonly used in MM calculations. The atomic charges for the QM atoms can be obtained by a population analysis scheme. Alternatively, there might be a sum of interactions with the QM nuclear charges plus the interaction with the electron density, which is an integral over the electron density. 

The remainder of the optimization output file displays the population analysis, molecular orbitals (if requested with Pop=Reg) and atomic charges and dipole moment for the optimized structure. 

This exercise will examine other ways of computing charges other than Mulliken population analysis. Since atomic charge is not a quantum mechanical observable, all methods for computing it are necessarily arbitrary. We ll explore the relative merits of various schemes for partitioning the electron density among the atoms in a molecular system. 

There is very little point in trying to obtain information from the 73 x 47 z= 3431 numbers that constitute the HF-LCAO coefficients for the occupied orbitals. Mulliken population indices are given next, together with Mulliken atomic charges (Figure 10.14). 

The Mulliken and Ldwdin methods give different atomic charges, but mathematically there is nothing to indicate which of these partitionings gives the best result. There are some common problems with all population analyses based on partitioning the wave function in terms of basis functions. 

The dipole, quadrupole etc. moments are in general not conserved, i.e. a set of population atomic charges does not reproduce the original multipole moments. 

Q Atomic charge (can be fractional), fitted or from population analysis... 

Population Analyses Population analyses are used to gain a detailed understanding of the electronic properties of a molecule. A common feature of most of these analytic tools is the definition of atomic charges. Because there is no... 

BOPs and atomic charges by population analysis are very sensitive to changes in the MO formulation and to the approximations (e.g. CNDO, EHT), and even to small basis set changes. 

An example of quantum mechanical schemes is the oldest and most widely used Mulliken population analysis [1], which simply divides the part of the electron density localized between two atoms, the overlap population that identifies a bond, equally between the two atoms of a bond. Alternatively, empirical methods to allocate atomic charges to directly bonded atoms in a reasonable way use appropriate rules which combine the atomic electronegativities with experimental structural information on the bonds linking the atoms of interest. A widely used approach included in many programs is the Gasteiger-Hiickel scheme [1]. 

Charge transfer and atomic electron population changes... 

The atomic charge q(Sl) is then simply obtained as — N(Sl), or the electron population subtracted from the charge of the nucleus inside the atomic basin. 

The volume, electron population, and charge of the whole molecule can also be obtained by the same method. Table 6.1 gives the atomic volumes, populations, and charges of the atoms in methanal. We see that the electron populations of the atoms and the corresponding charges are additive to four decimal places. 

Integration of these component peaks, with appropriate corrections applied for different photoionization cross-sections and inelastic mean free paths, gives the electron populations listed in Table 4. The atomic charges obtained are consistent... 

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