The same basic equations apply in the trajectory-based formulation, except that as stated, the quantum potential is not computed from Eq. ( ). To understand how Q is defined, it is first necessary to discuss the trajectory labelling coordinate C, in terms of which the PDE solution for the quantum trajectory ensemble is expressed as x(C, t). In principle, there is complete freedom in terms of how C is defined. One of the simplest choices is to take C to be the initial value of a given trajectory at time t = 0-i.e., C = x 0 = x(t = 0). Through probability conservation [i.e., Eq. ( )], one then obtains the following relation for the density at any time t: