  Models to account for the observed experimental results for low-energy
nuclear reactions in palladium-deuteride systems are presented along with
calculated results. The crucial idea is a mechanism of improved probability for
the needed penetration of the Coulomb barrier for a D-D reaction. This
facilitation occurs, in general, with the formation of D^- ions at special
frequency modes (e.g. via phonons) and, specifically for the laser-stimulated
case, with utilization of enhanced optical potential at a selected interface.
Both mechanisms may work individually, or together, to increase the probability
of barrier penetration.
