Linear Scaling Constrained Density Functional Theory in CONQUEST.

The constrained density functional theory (cDFT) formalism is implemented in the linear scaling density functional theory (DFT) code CONQUEST. This will enable the simulation of electron-transfer processes in large biologically and technologically relevant systems. The Becke weight population scheme is chosen to define the constraint, as it enables force components to be calculated both analytically and efficiently in a linear scaling code. It is demonstrated that the imposition of a constraint is not affected by the truncation of the density matrix. Demonstration calculations are performed on charge-separated excited states in small biphenyl molecules, and cDFT is found to produce accurate energy and geometry changes for this system. The capability of the method is shown in calculations on poly phenylene-vinylene oligomers and a hydrated DNA 10-mer.