Secular drift

Fig. 3.7 The energy for Euler s method applied to the 7-atom Lennard-Jones cluster is seen to exhibit a secular drift with the energy error growing approximately linearly in time...

The term secular growth in this context is a reference to the long-term growth of perturbations in celestial mechanics. For example, the precession of the Earth s polar axis occurs on a long period relative to its orbital motion and much longer period than its rotation, and so may be classed as a secular motion. In the context of molecular simulations, we use this to refer to accumulation of drift that takes the system steadily away from the energy surface. 

The uncertainties on the measured frequencies, either cOi and co+ or co+ and co, translate to an uncertainty on the mass m2 of the unknown ion determined from Equation 10.10. As shown in Figure 10.16, the relative uncertainty depends on the chosen reference measurement as well as the mass ratio p. For a mass ratio larger than 1.25, it appears to be advantageous to determine the mass from co+ and co, however, the difficulties in making a precise determination of cd are sufficiently severe that a mass measurement based on cOi and co+ is generally preferable, provided that error sources, which are more severe for mass measurements based on o), and C0+, for example, drift of the ion-trap frequency and RF-induced secular potentials, are sufficiently small. 

---