Quantum chemical molecular dynamics

Approximations can be consistent with the very high accuracy associated with traditional quantum chemistry provided that the approximations are variational. Variational approximations include the linear combination 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 variationally yields the Kohn-Sham-Roothaan equations. These ideas are illustrated in a derivation of accurate forces for quantum chemical molecular dynamics using the very efficient analytic Xa method. This method is used to follow directly the time evolution of collisions in a collinear (NO)(12) system and the excited-state dynamics of ICN and ICH3 photodissociation. (C) 1998 John Wiley & Sons, Inc.