ab inito Cyclic Cluster Model
The Cyclic Cluster Model (CCM) directly applies periodic boundary conditions (PBC) to a finite free cluster corresponding to a non-primitive unit cell. The interaction range of every atom within the cluster is defined by its Wigner-Seitz Cell (WSC) defined by the translation vectors. In contrast to conventional periodic models, the CCM is a Γ point approach and integration is carried out in real space.The challenge for the development of the CCM at ab initio level is the treatment of three- and four-center integrals. Special attention has to be paid to atoms at the border of WSCs, interactions have to be weighted correctly.
The ab initio Cyclic Cluster Model (AICCM) program is an object oriented, educational quantum chemical code written in the Python scripting language with C extensions. It depends on the Atomic Simulation Environment (ASE) for calculation setup and makes heavy use of Python modules (NumPy, SciPy, Matplotlib) and LAPACK routines. It is developed by Michael F. Peintinger and Thorsten Claff at the Mulliken Center for Theoretical Chemistry at the University of Bonn (Germany).