The CCM Problem Description Assistant





Systems

In this section, you should specify the type of system you are interested in, with particular focus on the phase of the system and its size or complexity.

Gas Phase System

Gas phase systems are typically isolated molecules. Computationally they are assumed to exist in a vacuum devoid of other systems. For computational considerations, gas phase molecules can be classified according to size. As a rough guideline, we have instituted a threefold partitioning scheme, with "small" being listed as those with fewer than 15 atoms, medium sized as being between 15 and 30, and large as being greater than 30.

Bulk Periodic System

Bulk periodic systems, by contrast, are assumed to be regular crystalline materials without appreciable voids or gaps. With larger unit cells, it is possible to approximate noncrystalline (i.e., amorphous) solids, although periodic boundary conditions will still be in effect. Because the 'size' of the system is actually infinite (due to system periodicity), one instead categorizes different prospective systems according to the complexity of the unit cell (the fragment of the bulk that is repeated periodically). By analogy with the gas phase system, we have chosen similar a partitioning scheme, with a simple unit cell being less than 15 unique atoms, a medium complexity case to be between 15 and 30, and a complex system to require treatment of more than 30 atoms.

Interface System

Interface systems are often treated via the same methods as are bulk solids, but typically have some major disruption of symmetry designed to approximate the junction of material of two different types or phases, which generally increases the complexity of the calculation significantly.

Defects

Defects also entail disruptions of symmetry within a continuous solid, but in this case are regarded as localized regions (i.e., point defects) of asymmetry surrounded by fairly regular bulk.

Adsorbate System

Adsorbate systems are similar to interfaces, but can be specifically characterized as individual molecular units attached to a (usually regular) solid surface.

Solvated System

Solvated systems entail individual molecules immersed in a fluid, represented either as a collection of discrete molecules (explicit solvation) or as a continuum medium (a collection of forces which approximate the collective effect of a large collection of solvent molecules on the solute molecule. One part of computational categorization entails a molecule size partitioning scheme identical to the gas phase system. The size of the desired solvation shell is also a consideration, however. For this, we have, as a rough guideline, chosen to call a system with 5 or fewer explicit solvent molecules "small", a system with between 5 and 30 to be medium, and greater than 30 to be large. If one wishes to use a continuum solvation model, however, computational complexity is independent of the solvation shell, and depends only on the size of the solute molecule and the specific choice of solvation model.

Mesoscale Systems

An additional category is provided for system type, namely "mesoscale" which refers to large systems which are approaching the sort of size where there is a question as to whether fully atomistic modeling is the best strategy. Many the properties treatable for smaller systems may not be feasible for systems of such size.