Large Scale Dynamics with Quantum Mechanical Forces: The Transfer Hamiltonian

The question of 'predictability' in multi-scale materials simulations is a
very
important one, since we want the results to be reliable, qualitatively or
quantitatively. The frist step in achieveing this largely depends upon having
a
source of accurate quantum chemical forces that underlie the classical
dynamics. However, unlike small molecules, where highly sophisticated and
predictive methods like coupled-cluster theory can be applied, for any
multi-scale simulation hundreds to thousands of atoms need to be described by
the quantum chemical methods with an efficientcy that permits tying to
dynamics. To solve this problem we have embarked upon the concept of a
'transfer Hamiltonian' which is formally defined by coupled-cluster theory
whose eigenfunction is a single determinant, yet whose energy and forces are
exact. Furthermore, it also permits the treatment of different electronic or
ionized states permitting optical properties and state specificity to be
described as part of the simulation. This transfer Hamiltonian can be
evaluated
purely from first principles, or from determining parameters that define the
Hamiltonian for the particular phenomena of interest. We will illustrate this
approach in comparison with others like density functional theory in problems
involving the fracture of silica including the presence of water.