Construction of accurate potential energy (PE) surfaces for molecular systems is one of the primary tasks performed by theoretical
physical chemists. Once in hand, these PE functions can be used to study the dynamics and spectroscopies,
as well as the structures and properties of molecular systems. This study focuses on approximating many-body electronic induction
in order to improve the accuracy of existing potentials and improve the efficiency of {em{ab initio}}
methods in order to allow “on-the-fly” energy and force evaluations in dynamical calculations.
The majority of the work reported here focuses on the solvated electron. We initiate a study aimed at understanding the effects of
explicitly including the (ultrafast) electron--solvent electronic induction, or polarization. We construct a single electron potential in which the coarse grained
electronic degrees of freedom of the solvent are treated
self-consistently along with the electronic wave function.
Predictions of the binding energy of an excess electron in water clusters obtained using this potential compare well to {em{ab initio}} electronic structure theories.
Subsequently, this potential was used
to investigate the behavior of the excess electron in liquid water. The explicit treatment of induction appears to have a minimal impact on the structure
and solvation dynamics of the excess electron (in the ground state) but does have a large impact on the vertical detachment energy and the
optical absorption spectrum. In these latter cases there is an abrupt change in the charge distribution of the excess electron.
In such cases the electronic response from the solvent can be large and should be taken into account.
The electronic response of the solvent occurs on the time scale of electronic excitation.
This introduces technical complications when solving for orthogonal eigenstates of this system since the model Hamiltonian
is state dependent. We describe a simple
method of solving this problem and discuss the possibility of generalizing this scheme to many-electron theories
(such and density functional and Hartree-Fock theories). This procedure may potentially enable the study of non-adiabatic excited state
relaxation dynamics including the electronic response of the solvent.
Construction of empirical potential energy surfaces, such as the one developed here, is a time intensive process and one questions
whether or not they have found the optimal set of parameters.
We would prefer to use accurate electronic structure theories to compute energies and forces. Of particular interest
is the use of {em{ab initio}} methods which offer a systematically improvable route to the exact energy.
Currently this is only feasible for small systems and short time scales. A class of algorithms called fragment methods are currently being
developed to extend these approaches to condensed phase environments. Our strategy has been to efficiently approximate
electronic induction and fold this into the description of the single molecular fragments. The fragments are then coupled
to one another through a version of symmetry adapted perturbation theory. This yields an accurate and efficient method that
scales linearly in the large system limit.