In Newton-X, this velocity adjustment can be made either in the direction of the nonadiabatic coupling vector (if available) or in any direction in the plane defined by the nonadiabatic coupling and gradient difference vectors. When these vectors are unknown, velocity adjustment is made in the direction of the momentum. This latter choice is not size-extensive and may lead to an artificial excess of back hoppings. This problem can be avoided by allowing hoppings to upper states only if the energy required is smaller than the nuclear kinetic energy per degree of freedom. The size-extensivity problem is already noticeable for medium-sized molecules. In reference [42], for example, reducing the kinetic energy available for back hopping was imperative to adequately describe the excited-state dynamics of pyrene (26 atoms) after excitation into S7.