Direct quantum chemistry covers a set of quantum chemical methods not using the Born-Oppenheimer representation. Direct quantum chemistry considers the motion of the nuclei and the electrons on the same time scales. The method therefore considers the molecular Hamiltonian as a whole without trying to solve separately the Schrödinger equation associated to the electronic molecular Hamiltonian. Though the method is non-adiabatic it is distinguishable from most non adiabatic methods for treating the molecular dynamics, which typically use the Born-Oppenheimer representation, but become non-adiabatic by considering vibronic coupling explicitly.
Direct quantum chemistry is applied in the modelling of high-speed atomic collisions, where the nuclear motion may be comparable or faster than the electronic motion.