where V describes the potential for the whole system. The eigenvalues of H as a function of R are called the adiabatic surfaces : ϕ ϕ n ( q ; R ) . Typically, q corresponds to the electronic degree of freedom, light atoms such as hydrogen, or high frequency vibrations such as O-H stretch. The forces in the molecular dynamics simulations are derived only from one adiabatic surface, and are given by: