Improved Initial Guess of the Density Matrix

One of the challenges associated with using a Gaussian basis set electronic structure code is that the convergence of the SCF depends on the quality of the initial guess. During this project, code has been written which allows the density matrix to be constructed based on smaller (usually simpler and less expensive) computations. There are several instances where this code may be useful, for example:

• Molecular crystals -- the initial guess electron density can be constructed based on the converged electronic structure of the constituent molecules.
• Macromolecules, e.g. DNA, proteins, polymers -- the initial guess electron density can be constructed from converged electronic structure of monomer units.
• Supercells, e.g. for studying defect science -- while this is a more complex use of the code requiring a little more thought, the bulk electron density can be used to construct the guess for the ``bulk like'' region of the supercell.