QUANTUM-CHEMICAL MOLECULAR-DYNAMICS

Approximations can be consistent with the very high accuracy associate d with traditional quantum chemistry provided that the approximations are variational. Variational approximations include the linear combina tion of atomic orbitals (LCAO) approximation of Hartree-Fock-Roothaan theory. In density functional theory, variationality usually requires that approximations be made in the expression for the energy which var iationally yields the Kohn-Sham-Roothaan equations. These ideas are il lustrated in a derivation of accurate forces for quantum chemical mole cular dynamics using the very efficient analytic Xa method. This metho d is used to follow directly the time evolution of collisions in a col linear (NO)(12) system and the excited-state dynamics of ICN and ICH3 photodissociation. (C) 1998 John Wiley & Sons, Inc.