Density-Derived Electrostatic and Chemical approaches
Net atomic charges have two primary applications (dual use):
(1) to quantify charge transfer between atoms in materials; this identifies cations and anions and
(2) to provide an electrostatic model in classical force fields using atomistic simulations (e.g., classical molecular dynamics or Monte Carlo simulations).
The older charge partitioning methods were not optimized for this dual use. For example, charges fit specifically to the electrostatic potential (CHELP, CHELPG, Merz-Kollman, etc.) often did not give any kind of reasonable chemical description for buried atoms.
The density-derived electrostatic and chemical (DDEC) methods are optimized to assign net atomic charges that give a good approximation both to the electrostatic potential surrounding the material as well as to the chemical charge states of atoms in materials. In other words, they are optimized for dual use.
A key design consideration in the DDEC family of methods is to create methods that work across an extremely broad range of material types including molecules, ions, nano-structured materials, metals, insulators, dense and porous solids, organometallics, and polymers for all chemical elements of atomic number 1 to 109.
Another key design consideration is that iterative processes to compute DDEC net atomic charges, atomic spin moments, and other atom-in-material properties should be rapid, robust, and converge to unique solutions. Several generations of improvements of the DDEC methods have been published. Unfortunately, there were some problems with the earliest DDEC approaches (e.g., DDEC1, DDEC2, and DDEC3) that caused non-unique convergence ('runaway charges') in some materials. The latest generation (DDEC6) fixes these convergence problems and is described in the following publications:
T. A. Manz and N. Gabaldon Limas, “Introducing DDEC6 atomic population analysis: part 1. Charge partitioning theory and methodology,” RSC Advances, 6 (2016) 47771-47801 DOI:10.1039/c6ra04656h
N. Gabaldon Limas and T. A. Manz, “Introducing DDEC6 atomic population analysis: part 2. Computed results for a wide range of periodic and nonperiodic materials,” RSC Advances, 6 (2016) 45727-45747 DOI:10.1039/c6ra05507a
T. A. Manz, “Introducing DDEC6 atomic population analysis: part 3. Comprehensive method to compute bond orders,” RSC Advances, 7 (2017) 45552-45581 (open access) DOI:10.1039/c7ra07400j
N. Gabaldon Limas and T. A. Manz, “Introducing DDEC6 atomic population analysis: part 4. Efficient parallel computation of net atomic charges, atomic spin moments, bond orders, and more,” RSC Advances, 8 (2018) 2678-2707 (open access) DOI:10.1039/c7ra11829e
The "density-derived" refers to atom-in-material properties (e.g., net atomic charges, atomic spin moments, bond orders, atomic multipoles, etc.) that are computed as functionals of the electron and spin density distributions. One can also imagine additional properties that are computed from the first-order density matrix or molecular orbitals that are not functionals of the electron and spin density distributions. These "orbital-derived" properties include spdfg populations of atoms in materials, projected density of states plots, bond order components assigned to individual orbitals, etc. Together, these "density-derived" and "orbital-derived" properties form the Standard Atoms in Materials Method (SAMM). In other words, recent generation DDEC (e.g., DDEC6) method is the "density-derived" part of the SAMM.
A key design consideration of the SAMM approach is that all of the various component methods should work together to provide a chemically consistent description of atoms in materials. This means the net atomic charges, atomic spin moments, atomic multipoles, bond orders, bond order components, spdfg populations, polarizabilities, dispersion coefficients, and projected density of states plots should be chemically compatible with each other. For example, when summing the individual populations of the spdfg subshell populations for spin-up and spin-down electrons in a magnetic material, these yield the prior-computed net atomic charges and atomic spin moments and have chemical consistency with the computed bond orders. Moreover, the "orbital-derived" properties from the SAMM method are designed to be chemically consistent with "density-derived" properties. For example, integrating the projected density of states (PDOS) curves for a particular atom in a material regenerates the prior-computed net atomic charges, atomic spin moments, and bond orders.